propagation: ThreeGppShadowing test outcome depends on applications module

When the propagation module is enabled alone (--enable-modules propagation) the code compiles correctly but the ns3::ThreeGppShadowingTestCase in three-gpp-propagation-loss-model-test-suite.cc fails (without crashing).

I was not able to understand exactly why, but I discovered by elimination that the test passes if (and seemingly only if!) the applications module is added (--enable-modules "propagation;applications").

Not a high priority issue, just to report the unexpected behavior.