Fix a bug where the TeamCity build only reports one of the test_runner runs

Summary:
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.

Test Plan:
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