Speed up test compilation when Ninja is in use

[hageboeck]

• Roottest tests were compiled sequentially when Ninja was in use, to avoid a problem (the commit doesn't mention details). Since that was 8 years ago, we should give parallel Ninja another try.

Difference on my machine (8 physical cores) running ctest -R meta -j16:

Old	New
760.82user 54.57system	654.64user 52.71system
6:31.81elapsed 208%CPU	1:52.29elapsed 629%CPU

Note:

• I tested this locally on Gentoo, but our CI is kind of blind to running with Ninja. Maybe it's worth to take one of the distros and make it compile with Ninja?

[github-actions]

Test Results

    20 files      20 suites   3d 23h 42m 3s ⏱️
 3 045 tests  3 045 ✅ 0 💤 0 ❌
59 223 runs  59 223 ✅ 0 💤 0 ❌

Results for commit d37b65f.

♻️ This comment has been updated with latest results.

[dpiparo]

That is a good idea. We need to 1) Add ninja to one of the container images 2) build with ninja for that platform - this can be one of the experimental builds (clang? nomodules? marchnative?)