libs.mk: simplify test_libvpx target

rename LIBVPX_TEST_BINS to LIBVPX_TEST_BIN and remove foreach usage.
this was a leftover from having multiple targets with their own (single)
object list; the use of LIBVPX_TEST_OBJS so widely makes extending these
loops difficult.

Change-Id: I61bda1b91acb43145609f04b8fe6e45ec4483e22