Bug 947442 - Assert if no results have been collected.

This cause an error to be reported in the test results.
Bug 947442 - The reporter mark as a failure the lack of results.
Bug 947442 - Address review comments.
Bug 947442 - more nits. Don't collect memstat if runner isn't "host"
Merge pull request #16147 from hfiguiere/bug947442

Bug 947442 - Assert if no results have been collected. r=julienw