MM-16506: conditionally build go.mod for plugins (#11430)

* MM-16506: conditionally build go.mod for plugins

Unless `GO111MODULE=off`, generate a `go.mod` that points at the local copy of `mattermost-server` to ensure plugin tests that compile source code on demand always test with the local copy of mattermost-server.  This also fixes an issue with early adopters of `GO111MODULE=on` on the server failing to find the right version of go-i18n dependencies.

* plugin: enable testlib resource management

* customize fileutils.Find* for testlib

plugin/main_test.go

@@ -12,7 +12,10 @@ import (
 var mainHelper *testlib.MainHelper
 
 func TestMain(m *testing.M) {
-	mainHelper = testlib.NewMainHelperWithOptions(nil)
+	var options = testlib.HelperOptions{
+		EnableResources: true,
+	}
+	mainHelper = testlib.NewMainHelperWithOptions(&options)
 	defer mainHelper.Close()
 
 	mainHelper.Main(m)