tests/bsim/run_parallel: Fail if an explicitly listed test is missing

Instead of just printing a warning, fail (return != 0 to the shell)
if a test is missing, or a pattern does not match any test.

Also, warn (and return != 0) if no tests are found when SEARCH_PATH
is provided.

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