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

Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc