The junit output of each successive test_runner.py run overwrites junit_results.xml.
This leads to a build failure, but tests reporting as passed: https://build.bitcoinabc.org/viewLog.html?buildId=16488&buildTypeId=BitcoinABC_Master_BitcoinAbcMasterAsan&tab=buildResultsDiv
It also leads to incorrect test failure reporting, making it impossible to track flaky or spurious test failures.
This diff fixes that by writing each run to it's own output file.