Shuffle render tests via --shuffle

[brunoabinader]

This PR adds a new parameter --shuffle that causes the render tests sequence to be shuffled. Also, it was not previously possible to force a specific test run sequence because all tests would end up being run in alphabetical file system sequence. This PR also enables a specific test run sequence to be specified - which is great in cases you want to repeat a shuffled test sequence.

Shuffling among specified tests:

$ node test/render.test.js --shuffle "icon-visibility" "text-visibility"
* passed text-visibility visible
* passed icon-visibility visible
* passed icon-visibility none
* passed text-visibility none

Shuffling all available tests:

$ node test/render.test.js --shuffle

Specified tests are now run in the same sequence they are specified via parameters:

$ node test/render.test.js "text-visibility"
* passed text-visibility none
* passed text-visibility visible
2 passed (100.0%)

$ node test/render.test.js "text-visibility/visible" "text-visibility/none"
* passed text-visibility visible
* passed text-visibility none
2 passed (100.0%)