Update all of the gradle.build files

Two main reasons:

1. Pull in AssertJ so that it is available in any test that wants to use it:
   ```
   testImplementation "org.assertj:assertj-core:3.15.0"
   ```
2. Update JUnit to 4.13 (which includes `assertThrows` among other things):
   ```
   testCompile "junit:junit:4.13"
   ```