Permit testing sans Valgrind
Only run the check_exec on Valgrind when present as the prefix to the
--valgrind-cmd argument. This lets us run the test suite without
Valgrind via:
./tests/runtests.py --valgrind-cmd ""
Signed-off-by: Alexander Scheel <ascheel@redhat.com>
Reviewed-by: Robbie Harwood <rharwood@redhat.com>
Merges: #243