clear TESTS variable between subdirs

Originally committed as revision 12866 to svn://svn.ffmpeg.org/ffmpeg/trunk

Makefile

@@ -50,7 +50,7 @@ $(PROGS): %$(EXESUF): %_g$(EXESUF)
 	cp -p $< $@
 	$(STRIP) $@
 
-SUBDIR_VARS := OBJS ASM_OBJS CPP_OBJS FFLIBS CLEANFILES DIRS
+SUBDIR_VARS := OBJS ASM_OBJS CPP_OBJS FFLIBS CLEANFILES DIRS TESTS
 
 define RESET
 $(1) :=