Improve logs for Sauce labs runs on Travis

[rsimha]

This PR does the following:

• Exposes the actual number of integration tests that pass, fail, or are skipped using the super-dots reporter.
• Prints a human readable summary of the tests run, with failure reasons, using the mocha reporter in minimal mode.
• In case of test failures, no longer prints a meaningless call stack, but instead, cleanly exits the test runner with a process.exit(exitCode).

Fixes #10482

[rsimha]

/to @choumx

[rsimha]

@choumx, @erwinmombay, this is now ready for review.

See https://travis-ci.org/ampproject/amphtml/jobs/255352838 for an example of what the sauce labs logs will look like in case of a failure.