Fix eunit not running if there are no beams

I'm not sure how to reproduce the issue some people experienced
(OS, make version maybe? who knows) but I got a report that this
fixes it. It's also definitely the "right" way to do this.

plugins/eunit.mk

@@ -52,8 +52,9 @@ eunit: test-build
 	$(gen_verbose) $(call erlang,$(call eunit.erl,fun $(t)/0),$(EUNIT_ERL_OPTS))
 endif
 else
-EUNIT_EBIN_MODS = $(notdir $(basename $(call core_find,ebin/,*.beam)))
-EUNIT_TEST_MODS = $(notdir $(basename $(call core_find,$(TEST_DIR)/,*.beam)))
+EUNIT_EBIN_MODS = $(notdir $(basename $(ERL_FILES) $(BEAM_FILES)))
+EUNIT_TEST_MODS = $(notdir $(basename $(call core_find,$(TEST_DIR)/,*.erl)))
+
 EUNIT_MODS = $(foreach mod,$(EUNIT_EBIN_MODS) $(filter-out \
 	$(patsubst %,%_tests,$(EUNIT_EBIN_MODS)),$(EUNIT_TEST_MODS)),'$(mod)')