Use CMocka generated JUnit reports where possible

Use CMocka generated JUnit reports where possible

Where applicable, use the more detailed CMocka generated JUnit
reports which include subtest results and timings instead of the
one generated by Meson.

Flaky tests also require retrying, so use a wrapper and mark them
with a environment variable. This is done to avoid the need to compute
an intersection of suites in Meson which is not supported out-of-the-box
(`meson test --suite=foo,bar` runs the union of foo and bar).