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>

presubmit_canned_checks.py

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