testsuite: Avoid using 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.

In GTK 3.24.35, if the environment is such that the label-sizing.ui
reftest happens to be successful, the overall result of the test suite
is failure. This seems unlikely to have been the intention.

Instead of using should_fail, 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
with a command like:

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

This arrangement is inspired by GNOME/glib!2987, which was contributed
by Marco Trevisan.

Signed-off-by: Simon McVittie <smcv@debian.org>

.gitlab-ci/test-docker-meson.sh

@@ -22,9 +22,22 @@ meson \
 cd _build
 ninja
 
+# Meson < 0.57 can't exclude suites in a test_setup() so we have to
+# explicitly leave out the failing and flaky suites.
 xvfb-run -a -s "-screen 0 1024x768x24" \
     meson test \
         --timeout-multiplier 4 \
         --print-errorlogs \
         --suite=gtk+-3.0 \
-        --no-suite=gtk+-3.0:a11y
+        --no-suite=flaky \
+        --no-suite=failing
+
+# We run the flaky and failing tests to get them reported in the CI logs,
+# but if they fail (which we expect they often will), that isn't an error.
+xvfb-run -a -s "-screen 0 1024x768x24" \
+    meson test \
+        --timeout-multiplier 4 \
+        --print-errorlogs \
+        --suite=flaky \
+        --suite=failing \
+    || true