Show full result when failing to match output in tests

[dirceu]

We seem to have a flaky test in main; the test failure only tells us that the output is empty, but it doesn't show anything about stderr or about the overall success.

This PR introduces and uses two new assertions that might make it easier to debug such test failures:

• assert_stdout_includes
• assert_stderr_includes

If I manually setup a failure on that flaky test, this is what we get now:

cli
  gem
    generate
      it must not include `rbi` definitions into `tapioca` RBI        FAIL (1.61s)
        ########## STDOUT ##########
        Requiring all gems to prepare for compiling...  Done
        
        Removing RBI files of gems that have been removed:
        
          Nothing to do.
        
          Compiled tapioca
              create  sorbet/rbi/gems/tapioca@0.9.2.rbi
        
        Checking generated RBI files...  Done
        
          Changed strictness of sorbet/rbi/gems/tapioca@0.9.2.rbi to `typed: false` (conflicting with DSL files)
        
        All operations performed in working directory.
        Please review changes and commit them.
        
        ########## STDERR ##########
        <empty>
        ########## STATUS: true ##########

[paracycle]

🙏