Fix test that fails in isolation

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'
```

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