Update run_tests to generate xml reports

[nitsakh]

Add ability to generate xml report for test results.

[jkleinsc]

FYI @nitsakh @alexeykuzmin: Results in CircleCI: https://circleci.com/gh/electron/libchromiumcontent/6405

[jkleinsc]

Also FYI @nitsakh and @alexeykuzmin our Mac and Windows CIs didn't have the yaml module installed on them, so they were throwing this error:

  File "script\run_tests", line 9, in <module>
    import yaml
ImportError: No module named yaml

I resolved this on Windows by running pip install pyyaml. I need to do the same for our Mac build agents. For example see: https://windows-ci.electronjs.org/project/AppVeyor/libchromiumcontent/build/1.0.1399/job/6ft5rp29rpe9beag

[alexeykuzmin]

@jkleinsc We have pyyaml as a submodule. In order to make import yaml work we only should do this:

from config import VENDOR_DIR

PYYAML_LIB_DIR = os.path.join(VENDOR_DIR, 'pyyaml', 'lib')
sys.path.append(PYYAML_LIB_DIR)
import yaml

[alexeykuzmin]

https://circleci.com/gh/electron/libchromiumcontent/6405 "2987 failures"

That's slightly more than I expected ) Do all of these tests fail locally? @nitsakh

[jkleinsc]

@alexeykuzmin in regards to the failures, I think its because we are running headless and xvfb wasn't running. I'll update to run with xvfb.

[alexeykuzmin]

@jkleinsc Yeah, I guess it requires the same setup we have for Electron.

[jkleinsc]

Closing in favor of #530 which continues this PR.