[flang] Prevent .mod files from being left by tests

test_symbols.sh and test_folding.sh were running the compiler in the
test directory (e.g. `<build>/test/semantics`). This meant that .mod
files were being left there because they weren't cleaned up.

Change to run the compiler in the temp directory created for each test.
It is deleted at the end so files aren't left behind.

Original-commit: flang-compiler/f18@4f9735ada7b142e7a013bede4a3d258b6620a573
Reviewed-on: https://github.com/flang-compiler/f18/pull/506