build/make/Makefile: add a 'test_*' default target
build/make/Makefile: add a 'test_*' default target

allows 'make test_libvpx', etc. some reworking of the makefiles would be
needed to avoid hard coding targets here.

Change-Id: I18982dbf691e7d36ab8bcf5934bab9340687b061
(cherry picked from commit 25085a6ac21fc8e2341b92e2f1f14d5a7fef30c6)

luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,mbebenita/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,smarter/aom