Further "unknown marks warning" improvements

[nicoddemus]

As discussed in #4935 and #4935 (comment):

• Add --strict-marks as an alias to --strict, and use --strict-marks in the documentation and examples;
• Remove the various marks that are used through testing: foo, bar, multiple spellings of parametrize, etc.
• After the above, we should try to get rid of UnknownMarksWarning in favor of PytestWarning; if not, we should then create more subclasses of PytestWarning for the other warnings emitted by pytest.
• Add integration tests.