test: "detect" the test number automagically

Specifying the test number manually is tedious and prone to errors (as
recently proven). Since we have all the necessary data to work out the
test number, let's do it automagically.

test/TEST-41-ONESHOT-RESTART/test.sh

@@ -6,4 +6,4 @@ TEST_DESCRIPTION="Test oneshot unit restart on failure"
 # shellcheck source=test/test-functions
 . "${TEST_BASE_DIR:?}/test-functions"
 
-do_test "$@" 41
+do_test "$@"