Fix failing test due to listen gem warning on Ruby 3.4

[mattbrictson]

The listen gem has an undeclared dependency on logger, which causes a warning to be printed in Ruby 3.4:

listen-3.9.0/lib/listen.rb:3: warning: logger was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0. Add logger to your Gemfile or gemspec.

This causes one of our integration tests to fail, because we expect no warnings to be printed when running mt.

This needs to be fixed upstream in the listen gem (a PR is already open: guard/listen#581). For now, work around the problem by ignoring this particular warning.