build: fix test-gc weakref build rule

Make the build rule depend on the build artifact (weakref.node) itself
rather than the directory it's built in.  Depending on the directory
means that a build failure won't trigger a rebuild on the next
invocation because the directory's timestamp has been updated.

This is a back-port of commit 1189571 from the master branch that
hopefully fixes the following CI error:

    executing:  make test/gc/node_modules/weak/build/
    make: *** No rule to make target `test/gc/node_modules/weak/build/'.
    Command exited with non-zero: make test/gc/node_modules/weak/build/
    Build step 'Execute NodeJS script' marked build as failure

Makefile

@@ -82,16 +82,16 @@ test-http1: all
 test-valgrind: all
 	$(PYTHON) tools/test.py --mode=release --valgrind simple message
 
-test/gc/node_modules/weak/build:
+test/gc/node_modules/weak/build/Release/weakref.node:
 	@if [ ! -f node ]; then make all; fi
 	./node deps/npm/node_modules/node-gyp/bin/node-gyp rebuild \
 		--directory="$(shell pwd)/test/gc/node_modules/weak" \
 		--nodedir="$(shell pwd)"
 
-test-gc: all test/gc/node_modules/weak/build
+test-gc: all test/gc/node_modules/weak/build/Release/weakref.node
 	$(PYTHON) tools/test.py --mode=release gc
 
-test-all: all test/gc/node_modules/weak/build
+test-all: all test/gc/node_modules/weak/build/Release/weakref.node
 	$(PYTHON) tools/test.py --mode=debug,release
 	make test-npm