enable non-GOPATH testing with different mattermost-server folder name (#12930)

* enable non-GOPATH testing

Tweak the testlib package to support finding the root without assuming the folder name of the repository.

Fixes: MM-19729

* fix web tests

web/web_test.go

@@ -12,8 +12,6 @@ import (
 	"path/filepath"
 	"testing"
 
-	"github.com/mattermost/mattermost-server/testlib"
-
 	"github.com/mattermost/mattermost-server/app"
 	"github.com/mattermost/mattermost-server/config"
 	"github.com/mattermost/mattermost-server/model"
@@ -97,11 +95,6 @@ func Setup() *TestHelper {
 }
 
 func (th *TestHelper) InitPlugins() *TestHelper {
-
-	if th.tempWorkspace == "" {
-		th.tempWorkspace, _ = testlib.SetupTestResources()
-	}
-
 	pluginDir := filepath.Join(th.tempWorkspace, "plugins")
 	webappDir := filepath.Join(th.tempWorkspace, "webapp")