autotests should work when using shadow builds

Currently, most tests assume that they are built in the source directory, and can get data from the current directory. The proper solution is to do it like the qimage test.

1. All tests that use source data should be changed to use the SRCDIR pattern

2. Is there a way to make this easier to achieve? It is ugly to have to put
DEFINES += SRCDIR=\\\"$$PWD\\\" into every .pro file.