T164 - Implement junit output support in test_runner.py
T164 - Implement junit output support in test_runner.py

Summary:
Test results are saved in test/functional/junit_results.xml (if you have better location, please propose it).

Here is part of output:

```
<?xml version="1.0" encoding="UTF-8"?>
<testsuite failures="0" id="0" name="bitcoin_abc_tests" skipped="1" tests="65" time="227" timestamp="2018-02-13T06:11:18.717714">
   <testcase classname="abandonconflict.py" name="abandonconflict.py" time="15">
      <system-out>2018-02-13 11:09:07.161000 TestFramework (INFO): Initializing test directory /tmp/bitcoin_test_runner_20180213_060723/abandonconflict_604
2018-02-13 11:09:19.598000 TestFramework (INFO): If balance has not declined after invalidateblock then out of mempool wallet tx which is no longer
2018-02-13 11:09:19.598000 TestFramework (INFO): conflicted has not resumed causing its inputs to be seen as spent.  See Issue #7315
2018-02-13 11:09:19.599000 TestFramework (INFO): 2489.99988480 -&gt; 2489.99988480 ?
2018-02-13 11:09:19.599000 TestFramework (INFO): Stopping nodes
2018-02-13 11:09:21.816000 TestFramework (INFO): Cleaning up
2018-02-13 11:09:21.819000 TestFramework (INFO): Tests successful
</system-out>
      <system-err/>
   </testcase>
   <testcase classname="abc-cmdline.py" name="abc-cmdline.py" time="8">
```

Unfortunately the test framework does not expose individual asserts/tests so we ouput one testcase per .py file (see abc-p2p-fullblocktest.py, which acutlaz runs 36 tests)

Refs T164

Test Plan: ./test/functional/test_runner.py

Reviewers: schancel, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: schancel, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T164

Differential Revision: https://reviews.bitcoinabc.org/D1089

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