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

testlib/resources_test.go

@@ -0,0 +1,15 @@
+package testlib
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFindDir(t *testing.T) {
+	t.Run("find root", func(t *testing.T) {
+		path, found := findDir(root)
+		assert.True(t, found, "failed to find root")
+		assert.NotEmpty(t, path)
+	})
+}