Fix Makefile for consistency

Author: jwbowler

Makefile

@@ -30,7 +30,7 @@ core-dev: generate
 # Shorthand for quickly testing the core of Terraform (i.e. "not providers")
 core-test: generate
 	@echo "Testing core packages..." && \
-		go test -tags 'core' $(TESTARGS) $(shell go list ./... | grep -v -E 'builtin|terraform/vendor')
+		go test -tags 'core' $(TESTARGS) $(shell go list ./... | grep -v -E 'terraform/(builtin|vendor)')
 
 # Shorthand for building and installing just one plugin for local testing.
 # Run as (for example): make plugin-dev PLUGIN=provider-aws