testlib: Blacklisting doesn't respect ClassName::function syntax

Autotest directories may contain more than one test class implementation, but only one BLACKLIST file.

qtestblacklist.cpp doesn't respect a syntax like tst_QWindow::isActive.
If test classes in the same directory have identical function names, or global blacklistings, all classes are affected by the BLACKLIST file.

This may lead to unwanted blacklistings.