stdlib modules should run via megatest (or megatest_stdlib), with special isMainModule handling

[timotheecour]

This is a followup from #9581

testament uses testStdlib to compile and run stdlib modules.
It relies on this logic:

if "when isMainModule" notin contents:
       testObj.spec.action = actionCompile

This is ok-ish but can have false positives (if appears in a comment), or false negatives, eg with when defined(osx) and isMainModule.

Instead of fixing this logic, a much better way is to extend the megatest idea from #9581 to stdlib modules.
This can be done as follows, without having to patch any stdlib module:

• add each stdlib file to megatest (minus a blacklist of non-joinable tests)
• patch compiler so isMainModule is equal to true when -d:isMainModuleIsAlwaysTrue is provided

That'll give the speedup benefit from #9581, and make that logic less fragile

note

on my machine,

nim c -r testament/tester pcat lib # 102 seconds
nim c -r testament/tester all # 390 seconds

so we can save roughly 25% with this (minus time to compile/run the megatest)

But this saving will become more significant as we address remaining issues, eg I have ideas for joining certain failing tests using the new --errorMax:N flag (will discuss in other issue)

[Araq]

I had the same plan so go ahead already. :-)

[Araq]

I changed my mind, megatest is bad enough already, I don't want even more of the same.