src/sage/doctest/forker.py: report verbose doctest success in CPU time

Since we're now warning about tests that take too many cpu-seconds (as
opposed to too many wall-seconds), it makes sense to report the time
taken in cpu-seconds as well. This allows you to better judge how
close a test is to being a "long" test. The output format remains
unchanged, for example,

  ok [1.02 s]

but now the "s" refers to cpu-seconds instead of wall-seconds. A
subsequent commit will disambiguate the two types of "s".

Issue: sagemathGH-32981

src/sage/doctest/forker.py

@@ -1370,13 +1370,13 @@ def report_success(self, out, test, example, got, *, check_duration=0):
             sage: FDS = FileDocTestSource(filename, DD)
             sage: doctests, extras = FDS.create_doctests(globals())
             sage: ex = doctests[0].examples[0]
-            sage: ex.walltime = 0.0
+            sage: ex.cputime = 0.0
             sage: DTR.report_success(sys.stdout.write, doctests[0], ex, '1764')
             ok [0.00 s]
         """
         # 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" % (example.walltime + check_duration))
+            out("ok [%.2f s]\n" % (example.cputime + check_duration))
 
     def report_failure(self, out, test, example, got, globs):
         r"""