[webkit-dev] iPad tests are now possible

[Simon Fraser]

With https://trac.webkit.org/changeset/204477 and some earlier groundwork, it's now possible to add
iPad-specific tests. Any test in a directory with an "ipad" suffix will be run in an iPad Air simulator,
both with DumpRenderTree and WebKitTestRunner.

When combined with "<!-- webkit-test-runner [ useFlexibleViewport=true ] -->", the test will run in
an iPad-sized web view, but this only works in WebKitTestRunner at present.

This is implemented by having webkitpy Ports expose "custom device classes". As tests are gathered,
any test in a directory with a suffix that matches a custom device class is gathered into a list of tests
that will be run after the rest of the tests, by setting up the testing environment for that particular
device class. So any port can specify custom device classes by simply declaring a CUSTOM_DEVICE_CLASSES array.

Let me know if you find any issues.

Simon