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

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.

a11ytests.test doesn't seem to be reliable enough to be used as a QA
acceptance criterion, and has been disabled as a build-time test in both
Gitlab-CI and Debian since 2019. a11ystate.test is not set up to be run
at build time at all, and has been marked as flaky on ci.debian.net
since 2018.

The rest of the testsuite/a11y directory seems to have been
reliable in practice, at least on ci.debian.net, so try leaving them
enabled as installed-tests.

In principle this could be made finer-grained by having a separate .test
file and a separate Meson test() for each .ui file, but that would
require more active maintenance of GTK 3.

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