[flang][driver] Refine tests for module search directories
This patch refactors include-module.f90:
* rename the test file as use-module.f90 to better highlight which
driver feature is being tested
* move tests for diagnostics to use-module-error.f90 (it tests that
`-J/-module-dir` can only be used once)
* make sure that `f18` is tested when `FLANG_BUILD_NEW_DRIVER` is
set to `Off`
* add tests for when all module files are successfully discovered and
loaded
With this patch, there should be a clear separation into 3 scenarios in
use-module.f90:
* Everything is OK
* One module file wasn't found (missing include path for
basictestingmoduletwo.mod)
* Two module files are found, but the test requires
`basictestingmoduleone.mod` from both `Inputs` and `Inputs/module-dir`.
Only the latter is found.
Reviewed By: tskeith
Differential Revision: https://reviews.llvm.org/D97197