Better test console reports; i18n XUnit results

Currently, when the doc and i18n tests fail in Jenkins, it's very hard to figure out what went wrong.  You can see at the end of the console log that tests failed, but it doesn't tell you which one(s), and the traceback is buried in lots of rake output garbage.

This addresses the issue by (1) adding section headers to the console output saying which test suite is running, (2) adding a list of tests that failed to the end of the test output.

Since these kinds of failures occur *before* running any unit tests, I think it makes more sense to have readable logs than to generate dummy XUnit reports.  However, the PR also adds xunit reports to the i18n tests, which should get picked up by Jenkins.

Changes:
* List which test suites failed
* Add section headings to test console output
* i18n tests generate XUnit reports
* Move i18n and docs test commands out of tests.rake

https://edx-wiki.atlassian.net/browse/TE-330

@jzoldak