Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)

Problem:    Memory test still fails on Cirrus CI.
Solution:   Allow for a tiny bit more tolerance in the upper limit.

Problem:    Memory test often fails on Cirrus CI.
Solution:   Allow for more tolerance in the upper limit.  Remove sleep.

Problem:    Memory test is flaky on FreeBSD.
Solution:   Add a short sleep before getting the first size.

Problem:    Tests use hand coded feature and option checks.
Solution:   Use the commands from check.vim in more tests.

Problem:    :let-heredoc does not trim enough.
Solution:   Trim indent from the contents based on the indent of the first
            line.  Use let-heredoc in more tests.

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes #4538)  Change the feature check into a command for
            consistency.

Problem:    Check for ASAN is not reliable.
Solution:   Check the version output. (Dominique Pelle, closes #4543)

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.

Problem:    Memory usage test is a bit too flaky.
Solution:   Adjust the tolerances a bit. (Christian Brabandt)