tests/qtest: tighten up the checks on clock_step

It is invalid to call clock_step with an implied time to step forward
as if no timers are running we won't be able to advance.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250207153112.3939799-7-alex.bennee@linaro.org>

system/qtest.c

@@ -708,10 +708,19 @@ static void qtest_process_command(CharBackend *chr, gchar **words)
         } else {
             ns = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL,
                                             QEMU_TIMER_ATTR_ALL);
+            if (ns < 0) {
+                qtest_send(chr, "FAIL "
+                           "cannot advance clock to the next deadline "
+                           "because there is no pending deadline\n");
+                return;
+            }
         }
         new_ns = qemu_clock_advance_virtual_time(old_ns + ns);
-        qtest_sendf(chr, "%s %"PRIi64"\n",
-                    new_ns > old_ns ? "OK" : "FAIL", new_ns);
+        if (new_ns > old_ns) {
+            qtest_sendf(chr, "OK %"PRIi64"\n", new_ns);
+        } else {
+            qtest_sendf(chr, "FAIL could not advance time\n");
+        }
     } else if (strcmp(words[0], "module_load") == 0) {
         Error *local_err = NULL;
         int rv;