[Flang] test_symbols.py module file fix

Due to how the LIT deals with module files,
this change stores and runs the scripts in
their own temporary directory to prevent
interference in-between different tests.
It also makes ``test_symbols.py`` be more
consistent with the other scripts.

Reviewed By: Meinersbur, awarzynski

Differential Revision: https://reviews.llvm.org/D107953