Adding changes to separate unit tests and integration tests (#13670)

* Introducing unit (not integration) tests for the app layer

* Initial support for unit tests at the API

* Adding unit tests support to the store layer

* Add unit tests support in commands

* Adding last tests needed for run unit tests properly

* Fixing govet

* Removing some duplication

* Fixing tests

* Fixing tests

* Not compiling test helpers with the main module for api

* Revert "Not compiling test helpers with the main module for api"

This reverts commit 36a199bbe0f7503f665f5d6c7b41c53aabc2e54f.

* Fixing tests

* Fixing unit tests

* More consistency between api4/apiteslib.go and app/helper_test.go

* Renaming things to make more obvious the new Setup functions purpose

* Reverting change in go.sum

* Start with empty mock for app layer

* Start with empty mock for api layer

* Start with empty mock for web layer

* Renaming SetupWithStoreMockConfig to SetupConfigWithStoreMock

* Fixing tests on web package

* Removing unnecesary function

api4/main_test.go

@@ -9,8 +9,6 @@ import (
 	"github.com/mattermost/mattermost-server/v5/testlib"
 )
 
-var mainHelper *testlib.MainHelper
-
 func TestMain(m *testing.M) {
 	var options = testlib.HelperOptions{
 		EnableStore:     true,
@@ -20,6 +18,5 @@ func TestMain(m *testing.M) {
 	mainHelper = testlib.NewMainHelperWithOptions(&options)
 	defer mainHelper.Close()
 
-	UseTestStore(mainHelper.GetStore())
 	mainHelper.Main(m)
 }