Readd run override (facebook#691)

Summary:
in order to support python 3.7, which has no `unittest.TestCase._callSetUp()` or `unittest.TestCase._callTearDown()` method, we add and remove alarms in the override of `unittest.TestCase.run()`

Pull Request resolved: facebook#691

Test Plan:
I tested this out in buck, with both an artificial timeout and an error and it gave normal output
```
test_fetch_mock_adhoc_fetcher: NoDataError: No metrics have availability overlapping with date range (2020-07-02, 2020-07-05).

Failures:

  1) ax.fb.utils.deltoid.tests.test_deltoid_apis.Deltoid3APIsTest: test_fetch_mock_adhoc_fetcher
    1) NoDataError: No metrics have availability overlapping with date range (2020-07-02, 2020-07-05).
      File "ax/fb/utils/deltoid/tests/test_deltoid_apis.py", line 785, in test_fetch_mock_adhoc_fetcher
        fetcher = MockDeltoidAdhocFetcher(trial=self.trial, metrics=self.metrics)
      File "ax/fb/utils/deltoid/mock.py", line 54, in __init__
        super().__init__(
      File "ax/fb/utils/deltoid/deltoid_apis.py", line 259, in __init__
        raise NoDataError(
Imports took: 41.1s! Profile with --import-profiler.           --_ |""---__
Executed 1 example in 41.7s:                                |'.|  ||  .    """|
  Successful: 0                                             | ||  || /|\""-.  |
  Failed: 1                                                 | ||  ||  |    |  |
  Skipped: 0                                                | ||  ||  |   \|/ |
  Not executed: 38                                          |."|  ||  --"" '__|
https://testslide.readthedocs.io/                              --" |__---"""
Failure: process exited with rc=1. (43.0s)
```
```
  test_fetch_mock_adhoc_fetcher: Exception: Test timed out at 1 seconds

Failures:

  1) ax.fb.utils.deltoid.tests.test_deltoid_apis.Deltoid3APIsTest: test_fetch_mock_adhoc_fetcher
    1) Exception: Test timed out at 1 seconds
      File "ax/fb/utils/deltoid/tests/test_deltoid_apis.py", line 786, in test_fetch_mock_adhoc_fetcher
        sleep(2)
      File "ax/utils/common/testutils.py", line 225, in signal_handler
        raise Exception(f"Test timed out at {self.MAX_TEST_SECONDS} seconds")
Imports took: 41.2s! Profile with --import-profiler.           --_ |""---__
Executed 1 example in 42.8s:                                |'.|  ||  .    """|
  Successful: 0                                             | ||  || /|\""-.  |
  Failed: 1                                                 | ||  ||  |    |  |
  Skipped: 0                                                | ||  ||  |   \|/ |
  Not executed: 38                                          |."|  ||  --"" '__|
https://testslide.readthedocs.io/                              --" |__---"""
Failure: process exited with rc=1. (44.6s)
```

Also tested both failure varieties in pytest and they look normal in 3.7 and 3.8, but those outputs are much longer.

Reviewed By: lena-kashtelyan

Differential Revision: D30843594

Pulled By: danielcohenlive

fbshipit-source-id: 5b9fdfb06e74a146eda222d03a6cfe3647bcde68

ax/utils/common/testutils.py

@@ -226,19 +226,18 @@ def signal_handler(signum: int, frame: Optional[FrameType]) -> None:
 
         super().__init__(methodName=methodName)
         signal.signal(signal.SIGALRM, signal_handler)
-        self.setUp_called = False
 
-    def _callSetUp(self) -> None:
+    def run(
+        self, result: Optional[unittest.result.TestResult] = ...
+    ) -> Optional[unittest.result.TestResult]:
         # Arrange for a SIGALRM signal to be delivered to the calling process
         # in specified number of seconds.
         signal.alarm(self.MAX_TEST_SECONDS)
-        self.setUp_called = True
-        super()._callSetUp()  # pyre-ignore[16]... it does
-
-    def _callTearDown(self) -> None:
-        # When seconds argument to alarm is zero, any pending alarm is canceled.
-        signal.alarm(0)
-        super()._callTearDown()  # pyre-ignore[16]... it does
+        try:
+            result = super().run(result)
+        finally:
+            signal.alarm(0)
+        return result
 
     def assertEqual(
         self,