Allow to sort by support

[SebastianZ]

Currently, the specs are sorted alphabetically thanks to @Zefling's work in https://github.com/LeaVerou/css3test/commit/1e5229d6b12295be0c61e5a382f93627584d5c9c.

Though to see which features can actually be used in a browser, it's also interesting to see how well the different specifications are supported.

Therefore there should be an option to change the order to be sorted by percentage of support.

To be able to do so, the execution of the tests needs to be split from their display.

Sebastian

[SebastianZ]

The split between test execution and display is covered by #215.

Sebastian