Since Qt 5.0.0 we never built the test plugins on Windows, because we
checked for the existence of QtCore[d]4.dll, and the actual test does
not complain if it cannot find any plugins.
Use the right conditions to check for debug/release Qt builds on
Windows. Use subdirs dependencies on every platform and then actually
pass the right variable to SUBDIRS. Clean up the pro file while we're at
it.
Change-Id: I099f30afd445fbf43dc5677d256ffe55b27639b3
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
bd4344a0bd