Bug 36756

The current unit tests don't compile inside of Qt. * tests.pri adds a tst prefix to all unit tests. qttest_p4.prf does the same thing, so we end up with tst_tst_qwebpage. This can be fixed by omitting the substitution for QTDIR builds. * tests.pri adds $$_PRO_FILE_PWD_/foo.cpp to SOURCES. For builds inside Qt the .pro file is separate and the sources are pulled in using VPATH. By adding $$_PRO_FILE_PWD_ to the VPATH we can use a relative name, so that for trunk builds tst_qwebpage.cpp is found in WebKit/qt/tests/qwebpage and for builds inside Qt, too, through VPATH.