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'
```

1 files changed, 1 insertions, 0 deletions

diff --git a/activesupport/test/file_update_checker_shared_tests.rb b/activesupport/test/file_update_checker_shared_tests.rb
index 100cbc9756..9c07e38fe5 100644
--- a/activesupport/test/file_update_checker_shared_tests.rb
+++ b/activesupport/test/file_update_checker_shared_tests.rb
@@ -21,6 +21,7 @@ module FileUpdateCheckerSharedTests
   end
 
   test 'should not execute the block if no paths are given' do
+    silence_warnings { require 'listen' }
     i = 0
 
     checker = new_checker { i += 1 }