In normal model project, the way to access parent AM from within nested AM is simple -

ParentAMImpl parentAM =  (ParentAMImpl)this.getRootApplicationModule();

However, the same approach doesn't help in JUnit model project. Use the following approach - 

Inside setUp() method --  ParentAM parentAM =  (ParentAM)Configuration.createRootApplicationModule(ROOT_AM, ROOT_AM_CONFIG);

Inside tearDown() method -- Configuration.releaseRootApplicationModule(parentAM, true);