test: set `lit.py` args in separate `setup_lit_args` (#82878)
Currently `test/CMakeLists.txt` can only set `SWIFT_LIT_ARGS` for all tests uniformly. This means that we can't have tests for Embedded Swift with a different set of params.
Let's refactor computation of `lit.py` options and arguments into a separate function, which means these params can set separately for different tests.
In this change we're only using it once, but in the future we anticipate another use of `setup_lit_args` specifically for Embedded Swift testing.
