add weblinks to all test output

[RainNapper]

Fixes #867

Test Plan

How do we know the code works?

1. Unit tests pass, and they verify the output and caught the expected changes
2. Ran locally with default configuration:

$ java -jar ./build/libs/flank.jar firebase test android run
version: local_snapshot
revision: 59772a4256e76ee65ca7b4c04f685891ad3043e5

...
(^ 0) [Tue Jun 23 19:00:32] ~/code/flank (rainnapper/weblink)
$ cat test_runner/results/2020-06-24_01-50-08.293000_MQbb/JUnitReport.xml
<?xml version='1.0' encoding='UTF-8' ?>
<testsuites>
  <testsuite name="NexusLowRes-28-en-portrait" tests="1" failures="0" flakes="0" errors="0" skipped="0" time="0.604" timestamp="2020-06-24T01:51:55" hostname="localhost">
    <testcase name="testPasses" classname="com.example.app.ExampleUiTest" time="0.604">
      <webLink>https://console.firebase.google.com/project/test-lab-test-project/testlab/histories/bh.36620515cd861620/matrices/8792094287040243811/executions/bs.545c75d14b4d4b63/testcases/1</webLink>
    </testcase>
  </testsuite>
</testsuites>

Checklist

• Documented
• Unit tested
• release_notes.md updated