tools: don't use f-strings in test.py

PR #43954 introduced the use of f-strings to `test.py` making the script
only work with python versions >= 3.6. Remove that use to make it
compatible again with older python versions.

PR-URL: #44407
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/test.py

@@ -227,7 +227,7 @@ def RunSingle(self, parallel, thread_id):
           if self.measure_flakiness:
             outputs = [case.Run() for _ in range(self.measure_flakiness)]
             # +1s are there because the test already failed once at this point.
-            print(f" failed {len([i for i in outputs if i.UnexpectedOutput()]) + 1} out of {self.measure_flakiness + 1}")
+            print(" failed %d out of %d" % (len([i for i in outputs if i.UnexpectedOutput()]) + 1, self.measure_flakiness + 1))
       else:
         self.succeeded += 1
       self.remaining -= 1