Failure information for prior failing test executions is omitted from XML reports

Given a flaky test such as:

```
def test_random():
assert bool(random.getrandbits(1))
```

The output of the JUnit XML contains no `` information when a test case fails and then subsequently passes.

It looks like this:

```

```

Other test frameworks and runners typically include this failure information like this so that automated tools can process flaky test information like this in a more structured format than plain-text logs.

```

def test_random():
> assert bool(random.getrandbits(1))
E assert False
E + where False = bool(0)
E + where 0 = <built-in method getrandbits of Random object at 0x7f93c1078610>(1)
E + where <built-in method getrandbits of Random object at 0x7f93c1078610> = random.getrandbits

test_sample.py:9: AssertionError
```

In order to avoid parsers mistaking the `` for deterministic, non-flaky failure, some test runners use ``. Even better, some test frameworks go so far as to mark the `` as `flaky="true"`. However, I'm not sure this library has that level of control to achieve these things, but if it does all the better for devs trying to fix flaky tests.