simplify executable_opts with new variable user_executable_opts

[smoors]

this PR adds a new variable user_executable_opts to the EESSI_Mixin class, which users can specify on the cmd line. advantages:

• gives users full control of the executable options: it will completely override the ones set in the test
• simplifies the tests: no more annoying check_custom_executable_opts(self, num_default=num_default) and has_custom_executable_opts
• consistency: some tests were using check_custom_executable_opts and has_custom_executable_opts, others were not.

[casparvl]

I love the idea of this one. It does indeed simplify the hooks.check_custom_executable_opts etc.

I think we should also remove that hook, as it shouldn't be used again. I think it's the only hook related to this specific change, right? (if there are more, all should probably be removed).

FYI: haven't tested yet... I'll do that once the changes are final

[casparvl]

Looks good to me. I tested this with:

reframe -c eessi/testsuite/tests/apps/tensorflow/tensorflow.py  -t CI -t 1_8_node --run --system=snellius:rome --setvar=user_executable_opts='--device gpu --intra-op-parallelism 16 --inter-op-parallelism 1'

Which (as intended) fails with an error because these nodes don't contain a GPU device. So: it respected the user_executable_opts I set.

I will double check with a regular run (setting no user_executable_opts) that the modified tests still run correctly before I merge, but all looks fine to me.

[casparvl]

Hm, also here a conflict. It's ready to be merged once the conflict is resolved :)