This test needs to have a require for the Listen gem or else it returns
an error when run by itself or with a certain order in the Active
Support tests. We use `silence_warnings` because Listen has some
warnings about private methods. It's already silenced when it's required
in Active Support EventedFileUpdateChecker.
```
1) Error:
EventedFileUpdateCheckerTest#test_should_not_execute_the_block_if_no_paths_are_given:
NameError: uninitialized constant EventedFileUpdateCheckerTest::Listen
test/evented_file_update_checker_test.rb:21:in `teardown'
```
Before
./Users/Juan/dev/rails/activesupport/test/file_update_checker_shared_tests.rb:20: warning: instance variable @tmpdir not initialized
After
No warnings
