It enables running pytests in cmake jobs, regardless of underlying build
tool choice (= makes it work with ninja.)
Also:
- drop pytest logic launching `make` and exiting in case of failure.
Maybe there is a better way and keep this functionality somehow, bind
it to a command-line option? make it fail softly?
- GHA/linux: invoke pytest via the build, not directly.
- autotools: add missing dummy runtests targets when cross-compiling.
Closes#15034
d82f9f965c