testsuite: Avoid using should_fail

• testsuite: Use separate setups for unstable tests instead of should_fail

  There are two possible interpretations of "expected failure": either the test must fail (exactly the inverse of an ordinary test, with success becoming failure and failure becoming success), or the test may fail (with success intended, but failure possible in some environments). Autotools had the second interpretation, which seems more useful in practice, but Meson has the first.

  Instead of using should_fail, we can put the tests in one of two new suites: "flaky" is intended for tests that succeed or fail unpredictably according to the test environment or chance, while "failing" is for tests that ought to succeed but currently never do as a result of a bug or missing functionality. With a sufficiently new version of Meson, the flaky and failing tests are not run by default, but can be requested by running a setup that does not exclude them, with a command like:

    meson test --setup=x11_unstable --suite=flaky --suite=failing

  As a bonus, now that we're setting up setups and their excluded suites programmatically, the gsk-compare-broadway tests are also excluded by default when running the test setup for a non-broadway backend.

  When running the tests in CI, --suite=gtk overrides the default exclude_suites, so we have to specify --no-suite=flaky and --no-suite=failing explicitly.

  This arrangement is inspired by glib!2987 (merged), which was contributed by Marco Trevisan.

• testsuite: Don't create .test files for flaky or failing tests

  These tests can be run manually, but are not suitable for use as an acceptance test, so let's not make frameworks like Debian's autopkgtest run these when they run ginsttest-runner in the most obvious way.

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Same idea as #5357 but for GTK 4.