Reland "Fix not-run-test detection."

This reverts commit 013e47be.

Reason for revert: This will be relanded as a warning, although the
bugs which the original change exposed should be fixed soon.

Original change's description:
> Revert "Fix not-run-test detection."
>
> This reverts commit 41691abe.
>
> Reason for revert: I thought https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3683378 would solve the problem but turns out this is the issue where it now reports it as a critical error.
>
> Original change's description:
> > Fix not-run-test detection.
> >
> > In crrev.com/c/2986400 the skip_shebang_check flag was added to address
> > the problem of some tests being run neither on Python 2 or Python 3. At
> > the same time reporting was added to warn if tests were run on neither
> > Python version.
> >
> > Unfortunately the reporting code was incorrect. A PresubmitPromptWarning
> > object was created but was not added to the list of results, so it had
> > no effect. Also the skipped tests name was added where a list was
> > expected which meant that each character of the test name showed up on
> > its own line.
> >
> > This change fixes the reporting. It also changes the format of the
> > report and makes it an error - a failure to run tests at all deserves
> > that severity level.
> >
> > A test run with the fixed check showed that no errors have crept in -
> > all tests were being run with one or the other. However the fix did
> > find a bug in an in-progress change I was working on.
> >
> > Bug: 1223478
> > Change-Id: Ibb44b5e60e7a7a5de08302f19ee4035cdfac5212
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3674199
> > Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
> > Commit-Queue: Fabrice de Gans <fdegans@chromium.org>
> > Reviewed-by: Fabrice de Gans <fdegans@chromium.org>
>
> Bug: 1223478
> Change-Id: If4395e90784c3f91692f398b4e5230a51139f78a
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3683105
> Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Reviewed-by: Bruce Dawson <brucedawson@chromium.org>

Bug: 1223478, 1330859
Change-Id: I6a0a78e70db77c9c5fda5bab27960cd4e71279e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3685349Reviewed-by: Fabrice de Gans <fdegans@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

presubmit_canned_checks.py

@@ -835,10 +835,9 @@ def GetUnitTests(input_api,
             message=message_type))
         test_run = True
       if not test_run:
-        output_api.PresubmitPromptWarning(
-            "Some python tests were not run. You may need to add\n"
-            "skip_shebang_check=True for python3 tests.",
-            items=unit_test)
+        results.append(output_api.PresubmitPromptWarning(
+            "The %s test was not run. You may need to add\n"
+            "skip_shebang_check=True for python3 tests." % unit_test))
   return results