test: wait until the unit finishes before checking the log

test: wait until the unit finishes before checking the log

Otherwise we might read an incomplete log and fail:

```
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 "/build/./test/test-exec-deserialization.py", line 152, in test_added_after
    self.check_output(expected_output)
  File "/build/./test/test-exec-deserialization.py", line 107, in check_output
    self.assertEqual(output, expected_output)
AssertionError: 'foo\n' != 'foo\nbar\n'
  foo
+ bar

----------------------------------------------------------------------
Ran 7 tests in 27.470s
```