Wait for guest connection before test restart

Even without a reboot, tmt still needs to verify the guest is up and
running. The reboot might be triggered beyond the control of tmt, and
that is fine, we just need to be sure we restart the test on guest
that's alive.

Related to #3284

tmt/steps/execute/__init__.py

@@ -331,6 +331,15 @@ def handle_restart(self) -> bool:
         else:
             self._restart_count += 1
 
+            # Even though the reboot was not requested, it might have
+            # still happened! Imagine a test configuring autoreboot on
+            # kernel panic plus a test restart. The reboot would happen
+            # beyond tmt's control, and tmt would try to restart the
+            # test, but the guest may be still booting. Make sure it's
+            # alive.
+            if not self.guest.reconnect():
+                return False
+
         self.logger.debug(
             f"Test restart during test '{self.test}'"
             f" with reboot count {self._reboot_count}"