test: call sync() before checking the test logs
    
    Otherwise we might hit a race where we read the test log just before
    it's fully written to the disk:
    
    ```
    ======================================================================
    FAIL: test_interleaved (__main__.ExecutionResumeTest.test_interleaved)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/root/systemd/test/test-exec-deserialization.py", line 170, in test_interleaved
        self.check_output(expected_output)
      File "/root/systemd/test/test-exec-deserialization.py", line 111, in check_output
        self.assertEqual(output, expected_output)
    AssertionError: 'foo\n' != 'foo\nbar\n'
      foo
    + bar
    ```
    
    With some debug:
    ```
    test_interleaved (__main__.ExecutionResumeTest.test_interleaved) ...
    Assertion failed; file contents just after the assertion:
    b'foo\n'
    
    File contents 5 seconds later:
    b'foo\nbar\n'
    FAIL
    ```
    
    Seen quite often in CentOS CI on the fast baremetal machines.