Fix testing plugin Makefile override warning

Author: raphael

plugins.mk

@@ -10,15 +10,20 @@
 # - build-examples: build the plugin examples binaries
 # - clean: clean the plugin examples binaries
 #
+# Set PLUGIN_CUSTOM_TEST=1 before including this file when the plugin Makefile
+# defines its own test target.
+#
 # Targets:
 # - "all" calls "gen", "test", "lint", "build-examples" and "clean"
 # - "lint" runs the linter and checks the code format using goimports
 # - "test" runs the tests
 
 all: gen test lint build-examples clean
 
+ifndef PLUGIN_CUSTOM_TEST
 test:
 	@go test ./...
+endif
 
 lint:
 	$(eval GO_FILES := $(shell find . -type f -name '*.go' ! -name '*.pb.go'))

testing/Makefile

@@ -7,6 +7,7 @@
 
 # Include the common plugin makefile
 PLUGIN_DIR := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
+PLUGIN_CUSTOM_TEST := 1
 include ../plugins.mk
 
 gen: