In #32842, I implemented a new ForwardCompatTestTrait in the phpunit-bridge to allow tests to run on the new php74-snapshot job, see #32289 This trait adds the void return type that phpunit 8 requires, while preserving compatibility with branch 3.4, which supports PHP 5.5. As a reminder, we target to make 3.4 run on PHP 7.4.

As of #32882, the phpunit-bridge is able to run phpunit 4.8 to 8 and PHP 5.5 to 7.4. It does so by polyfilling some missing methods and by patching the return type of setUp*/tearDown* methods when needed.

Now, there are failures we need to overcome on PHP 7.4 + phpunit 8, as reported here:
https://travis-ci.org/symfony/symfony/jobs/567908746

We need to implement polyfill methods in ForwardCompatTestTrait to allow fixing the deprecation warnings that phpunit 8 displays.

With #32887 in place, the process would be:

1. open a PR against branch 4.4 to make the phpunit-bridge polyfill a missing feature when needed
2. open a PR against branch 3.4 with the fix applied, and with the SYMFONY_PHPUNIT_BRIDGE_PR set to the number of the previous PR in .travis.yaml
3. we'll merge 1. once things are validated, and you'll have to revert the change on .travis.yaml before we merge 2.

Here is the list of things that phpunit 8 reports and should be fixed:

• Use assertContainsEquals when needed #32969 The optional $checkForObjectIdentity parameter of assert(Not)Contains() is deprecated and will be removed in PHPUnit 9. Refactor your test to use assert(Not)ContainsEquals() instead.
• Use assertEqualsWithDelta when needed #32968 The optional $delta parameter of assertEquals() is deprecated and will be removed in PHPUnit 9. Refactor your test to use assertEqualsWithDelta() instead.
• Use assertStringContainsString when needed #32971 Using assert(Not)Contains() with string haystacks is deprecated and will not be supported in PHPUnit 9. Refactor your test to use assertString(Not)ContainsString() or assertString(Not)ContainsStringIgnoringCase() instead.
• [PhpUnitBridge] Remove calls to deprecated function assertAttributeX #32923 readAttribute()/getObjectAttribute()/assertAttributeEquals()/assertAttributeSame() is deprecated and will be removed in PHPUnit 9.
• [PhpUnitBridge] Remove deprecated methods assertArraySubset #32909 assertArraySubset() is deprecated and will be removed in PHPUnit 9.
• [PhpUnitBridge] added polyfill for assertStringContainsString*() #32878 [PhpUnitBridge] add more assert*() polyfills #32907 polyfill assertString(Not)ContainsString(IgnoringCase), assertEqualsWithDelta and assert(Not)ContainsEquals
• [PhpUnitBridge] Remove @expectedException annotation #32875 The @ExpectedException, @expectedExceptionCode, @expectedExceptionMessage, and @expectedExceptionMessageRegExp annotations are deprecated. They will be removed in PHPUnit 9. Refactor your test to use expectException(), expectExceptionCode(), expectExceptionMessage(), or expectExceptionMessageRegExp() instead.
• [PhpUnitBridge] Fix deprecation assertInternalType #32846 assertInternalType() is deprecated and will be removed in PHPUnit 9. Refactor your test to use assertIsArray(), assertIsBool(), assertIsFloat(), assertIsInt(), assertIsNumeric(), assertIsObject(), assertIsResource(), assertIsString(), assertIsScalar(), assertIsCallable(), or assertIsIterable() instead.
• Add polyfill for TestCase::createMock() #32865 Declaration of ...::createMock($classname) must be compatible with PHPUnit\Framework\TestCase::createMock
• Ensure signatures for setUp|tearDown|setUpAfterClass|tearDownAfterClass methods in tests are compatible with phpunit 8.2 #32854 add void return types on branch 4.3+ where setup/teardown methods are still used
• Fix phpunit 8 compat to unlock testing Symfony on PHP 7.4 php-cache/integration-tests#102 do the same with 3rd party deps that we use

And fix PHP 7.4 deprecations:

• Fix tests/code for php 7.4 #32981 Invalid characters passed for attempted conversion
• Fix tests/code for php 7.4 #32981 Trying to access array offset on value of type [...]
• Fix tests/code for php 7.4 #32981 array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead
• [ProxyManagerBridge] Polyfill for unmaintained version #32992 Function ReflectionType::__toString() is deprecated
• [Yaml] PHP-8: Uncaught TypeError: abs() expects parameter 1 to be int or float, string given #32862 Uncaught TypeError: abs() expects parameter 1 to be int or float, string given
• [Yaml] PHP-8: Uncaught TypeError: abs() expects parameter 1 to be int or float, string given #32862 PHP-8: Uncaught TypeError: abs() expects parameter 1 to be int or float, string given
• Skip tests that fatal-error on PHP 7.4 because of missing parent classes #32864 skip tests that are fatal on PHP 7.4 for now - we can work on them later
• Fix using array_key_exists() on objects, it is deprecated in PHP 7.4 twigphp/Twig#3097 fix using array_key_exists() on objects in Twig
• Fix "access array offset on value of type null" PHP 7.4 notices doctrine/orm#7785 Remove imprecise T_IDENTIFIER capture groups, leading to simpler token sequences doctrine/annotations#258 Trying to access array offset on value of type [...]

Then:

• Fix deprecations on 4.3 #33000 same on 4.3
• same on 4.4/master
• fix PHP 7.4 breaks resiliency when loading classes with missing parents #32995

I'm opening this issue to ask for help to the community, this is a lot of work that can be split efficiently I believe.