src/sage/doctest/forker.py: specify walltime in sage -t --verbose output

Now that cpu seconds are used for --warn-long, we should disambiguate
wherever wall seconds are still used. One such place is in the verbose
test runner output, which outputs (for example)

  ok [3.83 s]

after a test that took 3.83 wall seconds. This commit changes that
output to

  ok [3.83s wall]

which is unmistakable.

src/sage/doctest/forker.py

@@ -1337,12 +1337,12 @@ def report_success(self, out, test, example, got, *, check_timer=None):
             sage: check.walltime = 2.71
             sage: DTR.report_success(sys.stdout.write, doctests[0], ex, '1764',
             ....:                    check_timer=check)
-            ok [3.83 s]
+            ok [3.83s wall]
         """
         # We completely replace doctest.DocTestRunner.report_success
         # so that we can include time taken for the test
         if self._verbose:
-            out("ok [%.2f s]\n" %
+            out("ok [%.2fs wall]\n" %
                 (example.walltime + check_timer.walltime))
 
     def report_failure(self, out, test, example, got, globs):