bsim tests: run_parallel: Allow providing tests explicitly

Add the options to provide a file with a list of tests
to run with a possible TESTS_FILE variable,
or to provide the actual list of tests directly
in TESTS_LIST.

If a file is provided, comments (#) and empty lines will be
skipped.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>