Fixed broken UnitTest build

Build files were broken for unit test. Seems to difficult to try to kep
qgroundcontrol.pro and qgcunittest.pro in sync with each other. Instead
I made qgcunittest.pro just include qgroundcontrol.pro. This required
some tweaks to allow TARGET to be set from the outside as well as to
leave out main.cc. Also needed to modify qgroundcontrol.pri to use
TARGET instead of hardcoding app name. This way it can build either
qgroundcontrol.pp or qgcunittest.app. At this point only works on osx.