Include .php tests in DumpRenderTree2.

I've manually checked the existing LayoutTests for false positives.
There are non-test .php files, but they're all in "resources"
subdirectories, so they should be caught by isTestDir(). Added an
extra @warning to isTestFile() just in case.

Bug: 3164771
Change-Id: Ib282acf46f6e839439ce521c4b2cb5820fcd263c