subtree: update `contrib/subtree` `test` target

The intention of this change is to align with how the top-level git
`Makefile` defines its own test target (which also internally calls
`$(MAKE) -C t/ all`). This change also ensures the consistency of
`make -C contrib/subtree test` with other testing in CI executions
(which rely on `$DEFAULT_TEST_TARGET` being defined as `prove`).

Signed-off-by: Victoria Dye <vdye@github.com>

contrib/subtree/Makefile

@@ -94,7 +94,7 @@ $(GIT_SUBTREE_TEST): $(GIT_SUBTREE)
 	cp $< $@
 
 test: $(GIT_SUBTREE_TEST)
-	$(MAKE) -C t/ test
+	$(MAKE) -C t/ all
 
 clean:
 	$(RM) $(GIT_SUBTREE)