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Description

Under some situations which were not entirely clear to me, the test_worktree.rb tests occasionally fail.

It ends up that this is because test_thread.rb tests leak setting the global ENV['GIT_DIR'] value.

This PR makes the following changes:

• test_worktree.rb tests were rewritten and do not use the test fixture. This makes the tests much easier to understand since you can see the setup for each test.
• The tests/files/worktree test fixture files are not needed so those files (270+ files!) were deleted.
• In test_thread_safety.rb, a teardown was added to reset the environment variables. This is a stop gap measure until Git::Lib#command can be rewritten to use a method that does not depend on the global ENV variable (currently it uses backticks to run git commands).
• In test_helper.rb, the Test::Unit::TestCase#in_temp_dir was changed to call File#realpath on the temp directory that it returns to mimic what this gem does internally.