core: rename unit_{start_limit|condition|assert}_test() to unit_test_xyz()

Just some renaming, no change in behaviour.

Background: I'd like to add more functions unit_test_xyz() that test
various things, hence let's streamline the naming a bit.

src/core/timer.c

@@ -607,7 +607,7 @@ static int timer_start(Unit *u) {
                 return -ENOENT;
         }
 
-        r = unit_start_limit_test(u);
+        r = unit_test_start_limit(u);
         if (r < 0) {
                 timer_enter_dead(t, TIMER_FAILURE_START_LIMIT_HIT);
                 return r;