Use central rpath.pri for setting RPATHs and qttestrpath.pri for tests.
Simplify install names of plugins and libraries on macOS to be just
@rpath/libName, which follows convention better and makes setting up
RPATHs easier.
Preparation for moving tools one directory level down on macOS, to
be able to add a qt.conf for the tools generically.
Task-number: QTCREATORBUG-23120
Change-Id: I16625d48904abd3a7f4c2ad7bbba5916cdc400cd
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>