Waiting after touching the file system is a concern of
`EventedFileUpdateCheckerTest`.  Therefore, only call `wait` inside
`EventedFileUpdateCheckerTest`.  This avoids calling `wait` an extra
time when calling `touch`.

Before:

    $ bin/test test/evented_file_update_checker_test.rb test/file_update_checker_test.rb

    Finished in 43.357019s, 0.9918 runs/s, 2.5371 assertions/s.
    43 runs, 110 assertions, 0 failures, 0 errors, 0 skips

After:

    $ bin/test test/evented_file_update_checker_test.rb test/file_update_checker_test.rb

    Finished in 34.351007s, 1.2518 runs/s, 3.2022 assertions/s.
    43 runs, 110 assertions, 0 failures, 0 errors, 0 skips