build: fail if --enable-tests and no nunit found (bgo#664887)
build: fail if --enable-tests and no nunit found (bgo#664887)

As tests are disabled by default, using the flag --enable-tests
at configure time should fail (and not just warn) and stop the
build, to let the developer know that the flag could not be
applied successfully.

ixfalia/banshee,Carbenium/banshee,GNOME/banshee,stsundermann/banshee,Carbenium/banshee,babycaseny/banshee,Carbenium/banshee,arfbtwn/banshee,babycaseny/banshee,stsundermann/banshee,GNOME/banshee,stsundermann/banshee,stsundermann/banshee,babycaseny/banshee,babycaseny/banshee,ixfalia/banshee,stsundermann/banshee,GNOME/banshee,GNOME/banshee,arfbtwn/banshee,babycaseny/banshee,babycaseny/banshee,GNOME/banshee,arfbtwn/banshee,Carbenium/banshee,ixfalia/banshee,babycaseny/banshee,arfbtwn/banshee,ixfalia/banshee,arfbtwn/banshee,Carbenium/banshee,stsundermann/banshee,GNOME/banshee,arfbtwn/banshee,ixfalia/banshee,stsundermann/banshee,ixfalia/banshee,GNOME/banshee,Carbenium/banshee,babycaseny/banshee,arfbtwn/banshee,stsundermann/banshee,arfbtwn/banshee,GNOME/banshee,ixfalia/banshee,ixfalia/banshee