[wptrunner] Display reruns starting from `1/N` to `N/N` (#43155)

Instead of `0/N` to `(N-1)/N`, which looks incomplete.
diff --git a/tools/wptrunner/wptrunner/testrunner.py b/tools/wptrunner/wptrunner/testrunner.py
index f29e902..2d81cab 100644
--- a/tools/wptrunner/wptrunner/testrunner.py
+++ b/tools/wptrunner/wptrunner/testrunner.py
@@ -640,7 +640,7 @@
         self.recording.set(["testrunner", "test"] + self.state.test.id.split("/")[1:])
         self.logger.test_start(self.state.test.id, subsuite=self.state.subsuite)
         if self.rerun > 1:
-            self.logger.info("Run %d/%d" % (self.run_count, self.rerun))
+            self.logger.info(f"Run {self.run_count + 1}/{self.rerun}")
             self.send_message("reset")
         self.run_count += 1
         if self.debug_info is None: