test: check for the output file in a loop
    
    This should make the test faster on fast machines and more reliable on
    slower/under-load machines, where the 4 sec sleep wasn't sometimes enough.
    
    Spotted on C8S machines under load:
    
    ```
    test_added_after (__main__.ExecutionResumeTest) ... FAIL
    test_added_before (__main__.ExecutionResumeTest) ... ok
    test_interleaved (__main__.ExecutionResumeTest) ... ok
    test_issue_6533 (__main__.ExecutionResumeTest) ... ok
    test_no_change (__main__.ExecutionResumeTest) ... ok
    test_removal (__main__.ExecutionResumeTest) ... ok
    test_swapped (__main__.ExecutionResumeTest) ... ok
    
    ======================================================================
    FAIL: test_added_after (__main__.ExecutionResumeTest)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "./test/test-exec-deserialization.py", line 101, in check_output
        with open(self.output_file, 'r') as log:
    FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmpjnec1dj4'
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "./test/test-exec-deserialization.py", line 150, in test_added_after
        self.check_output(expected_output)
      File "./test/test-exec-deserialization.py", line 104, in check_output
        self.fail()
    AssertionError: None
    
    ----------------------------------------------------------------------
    Ran 7 tests in 44.270s
    ```