tests: remove WarnTest

We would raise this if a test didn't return a result code. AFAICT
this can only occur if there is a logic error in the test harness
itself.

I don't think it is worth the code complexity to distinguish this
failure scenario from a regular test failure.

tests/run-tests.py

@@ -717,8 +717,6 b' class Test(unittest.TestCase):'
                 # test we "ran", but we want to exclude skipped tests
                 # from those we count towards those run.
                 result.testsRun -= 1
-            except WarnTest as e:
-                result.addWarn(self, str(e))
             except ReportedTest as e:
                 pass
             except self.failureException as e:
@@ -788,7 +786,7 b' class Test(unittest.TestCase):'
         elif ret == 'timeout':
             self.fail('timed out')
         elif ret is False:
-            raise WarnTest('no result code from test')
+            self.fail('no result code from test')
         elif out != self._refout:
             # Diff generation may rely on written .err file.
             if (ret != 0 or out != self._refout) and not self._skipped \
@@ -1481,9 +1479,6 b' class TTest(Test):'
 
 iolock = threading.RLock()
 
-class WarnTest(Exception):
-    """Raised to indicate that a test warned."""
-
 class ReportedTest(Exception):
     """Raised to indicate that a test already reported."""