SpawnIsolateMixin makes underlying errors not visible in traceback

[a-glapinski]

Using bigflow.testing.SpawnIsolateMixin mixin in the test case (like shown in the docs) makes underlying, actual errors not visible in the traceback. Regardless of the underlying exception (which can be discovered after removing SpawnIsolateMixin from the test case), RuntimeError: Isotalted process was unexpectedly terminated is always being printed in the traceback.

Here is an example:

2023-11-08T11:51:18.8312970Z =================================== FAILURES ===================================
2023-11-08T11:51:18.8314516Z ________ DatasetWorkflowTestCase.test_load_something _________
2023-11-08T11:51:18.8425321Z [gw19] linux -- Python 3.9.18 /opt/hostedtoolcache/Python/3.9.18/x64/bin/python
2023-11-08T11:51:18.8426349Z 
2023-11-08T11:51:18.8427441Z self = <test.some_service.test_something.DatasetWorkflowTestCase testMethod=test_load_something>
2023-11-08T11:51:18.8429540Z result = <TestCaseFunction test_load_something>
2023-11-08T11:51:18.8430959Z msg = 'Isotalted process was unexpectedly terminated'
2023-11-08T11:51:18.8431725Z 
2023-11-08T11:51:18.8432233Z     def _fail_into_result(self, result: unittest.TestResult, msg):
2023-11-08T11:51:18.8433594Z         """Include error with the traceback into `TestResult`"""
2023-11-08T11:51:18.8434627Z         try:
2023-11-08T11:51:18.8435779Z >           raise RuntimeError(msg)
2023-11-08T11:51:18.8436880Z E           RuntimeError: Isotalted process was unexpectedly terminated
2023-11-08T11:51:18.8437714Z 
2023-11-08T11:51:18.8438930Z /opt/hostedtoolcache/Python/3.9.18/x64/lib/python3.9/site-packages/bigflow/testing/isolate.py:102: RuntimeError