tests: replace helpers.enableExceptInTests with module option

[MattSturgeon]

As discussed on matrix, the current helpers.enableExceptInTests is problematic for a couple reasons:

• It only works with mkTestDerivationFromNixvimModule, not mkTestDerivationFromNvim
• We need to tell helpers whether or not it'll be used for tests

This resolves those, however it has its own issues:

• mkTestDerivationFromNvim effectively evaluates the modules twice (once before being called, then again internally)
  • I guess this is no different from evaluating a standalone build in one place and then again separately using mkTestDerivationFromNixvimModule
• If tests: move derivation to module-option & minor cleanup #1989 is merged, users can bypass the test helpers and access config.test.derivation directly anyway
• enableExceptInTests is removed immediately, with no grace period for the deprecation

I wonder if it'd be better to not provide any direct support for distinguishing between test/non-test contexts?

Or perhaps we should double down on the evalNixvimTest function proposed in #1989's description and use that to set the context?

See #1095