Make scores for partial runs clearer

[cknaap]

If some user runs a partial test on tests that happen to fail, a server could get a very low score and keep that until someone else runs a complete test (or the opposite; run only a subset that succeeds).
For novice users it is very unclear what the score is based on, plus it is used for the ranking on the homepage without any weight on the nr. of tests run.

Suggestions:

• ranking on the homepage only on full test runs
• separate list for partial runs, mentioning the % of tests that were run
• add clear explanation of the scores, and make it stand out.