test-execute: fix killing of unit processes

Let's use the correct wrapper for calling unit vtable functions. Let's
make sure we always use the right wrappers, and never bypass them
needlessly.

Moreover use SIGKILL rather than "9" as signal name. Let's not be
needlessly cryptic.

Follow-up for: f7f8e8cbb9

src/test/test-execute.c

@@ -54,7 +54,7 @@ static void check(Manager *m, Unit *unit, int status_expected, int code_expected
                 n = now(CLOCK_MONOTONIC);
                 if (ts + timeout < n) {
                         log_error("Test timeout when testing %s", unit->id);
-                        r = UNIT_VTABLE(unit)->kill(unit, KILL_ALL, 9, NULL);
+                        r = unit_kill(unit, KILL_ALL, SIGKILL, NULL);
                         if (r < 0)
                                 log_error_errno(r, "Failed to kill %s: %m", unit->id);
                         exit(EXIT_FAILURE);