Fix a bug where the TeamCity build only reports one of the test_runner runs
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.
See the TeamCity build for this diff: https://build.bitcoinabc.org/viewLog.html?buildId=16722&buildTypeId=BitcoinABC_BitcoinAbcStaging&tab=testsInfo
In the "View:" dropdown, select tests without grouping by name
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3691