Implemented make test

make test in project root will recurse to the t/ subdirectory and run
make all there.

Makefile

@@ -120,6 +120,9 @@ diff.o: $(LIB_H)
 strbuf.o: $(LIB_H)
 gitenv.o: $(LIB_H)
 
+test:
+	make -C t/ all
+
 clean:
 	rm -f *.o mozilla-sha1/*.o ppc/*.o $(PROG) $(LIB_FILE)