Enable `cling-test` for nightly, PR builds

[SimeonEhrig]

I have question. Does roottest runs cmake --build . --target cling-test when the cling interpreter code is changed? From time to time I see, that commits are merged regardless of whether the CI fails.
If I understand Cling's workflow correctly, changes are integrated and tested in ROOT and automatically streamed down to Cling after the merge in ROOT. So, if roottest does not run the integrated tests in cling, it is possible that the Cling CI is bypassed, or I'm wrong?

[Axel-Naumann]

cling-test is part of the test suite run with ctest if ROOT is configured with -Dclingtest=On. You can trigger Jenkins to do that by saying @phsft-bot build with flags -Dclingtest=On. But I agree it would make sense to turn it on by default for PR and nightly builds.

[SimeonEhrig]

@Axel-Naumann That sounds like a great idea.

[dpiparo]

@hahnjo @devajithvs @vgvassilev I can enable these tests for 2-3 platforms in master: what do you think?

[vgvassilev]

Sounds good to me.

[hahnjo]

clingtest exports many more symbols from libCling, for testing purposes (see the code in interpreter/CMakeLists.txt). This means our (patched) LLVM and Clang will interfere with other LLVM installations used by dependent libraries. IIRC this can make tests fail!

[dpiparo]

What then could be a good way to make progress about this issue?