Add a test matrix for E2E tests on each CMS
The test-runs which feed into the release status checking are (entirely?) based on D7. Of course, most of the test-coverage is CMS independent (
headless), but the E2E suite is an exception. Consequently, our release-process is missing out on available signals for WordPress and Backdrop (and maybe D6/D8/J).
I recently noticed that
E2E_Extern_RestTest on Backdrop has been reliably producing failures for several releases (at least 5.7 - 5.11). If the matrix were showing E2E results on the various CMS's, then I'm pretty sure we would've nipped this a while ago.