Add percentage of subtests to wpt.servo.org

[mrego]

Right now people are usually confused when they compare the results at wpt.servo.org vs wpt.fyi. For example:

Test suite	wpt.servo.org	wpt.fyi
All	62%	78.7% (1527155 / 1939497)
/css	68%	49.6% (81871 / 165034)
/css/CSS2	84.8%	86.5% (7423 / 8578)

We've added an explanation:

Scores are calculated as percentages of total enabled tests within the suite that pass. A passing test with no subtests gets a score of 1 while a test with subtests gets a score between 0 and 1 representing the fraction of passing subtests within that test. This is different from the percentages on wpt.fyi which is calculated by giving equal weight to both top-level tests and subtests.

But still it'd be useful if we can also show the percentage of subtests in wpt.servo.org, so we'll have 2 numbers for each folder (we could even have 2 lines in the chart).