Support "error" failures; add Github Actions outputs

[jcourteau]

Bazel generates JUnit-like output, but it uses the error key instead of failure. It also does some other silly things with test aggregation, but I didn't dive into that with this PR.

Also added some Github Actions outputs.

Changes:

• support the testsuites.testsuite[].testcase[].error XML node in addition to testsuites.testsuite[].testcase[].failure
• add Github Actions outputs for passed/failed/skipped/total tests
• added other outstanding features to the README
  • added the fact that the default output is now job summary
  • added docs for show
• added docs for new outputs
• added a Bazel Junit test output file
• added test coverage for the above

[mbien]

this would fix the issue I filed: #9

[jcourteau]

Updated the PR:

• rebased off of the latest changes
• bump actions/core to 1.10; setOutput in the previous versions is deprecated
• bump mocha to resolve a "high" risk vulnerability in a dependency, as reported by NPM.

[fpfaff]

any update on this? The bump of actions/core to the latest version would be nice to get rid of GitHub's warnings when using this action.

[ethomson]

Thanks @jcourteau -- merged!