Auth: Do not search for the user twice (#18366)

* Auth: Do not search for the user twice

Previously `initContextWithBasicAuth` did not use `LoginUserQuery`, doing
`GetUserByLoginQuery` only i.e. looking user in DB only, things changed when
this function started to check LDAP provider via `LoginUserQuery` (#6940),
however, this request was placed after `GetUserByLoginQuery`, so we first
looking in DB then in the LDAP - if LDAP user hasn't logged in we will
not find it in DB, so `LoginUserQuery` will never be reached.

`LoginUserQuery` request already performs `GetUserByLoginQuery`
request in correct sequence. So we can just remove redundant request.

* Correct sequence execution during authentification &
  introduce tests for it

* Move basic auth tests to separate test file, since main test file already
  pretty large

* Introduce `testing.go` for the middleware module

* Remove redundant test helper function

* Make handler names more explicit

Ref 5777f65d05
Fixes #18329

* Auth: address review comment

pkg/middleware/middleware.go

@@ -163,24 +163,11 @@ func initContextWithBasicAuth(ctx *models.ReqContext, orgId int64) bool {
 		return true
 	}
 
-	loginQuery := models.GetUserByLoginQuery{LoginOrEmail: username}
-	if err := bus.Dispatch(&loginQuery); err != nil {
-		ctx.Logger.Debug(
-			"Failed to look up the username",
-			"username", username,
-		)
-		ctx.JsonApiErr(401, errStringInvalidUsernamePassword, err)
-
-		return true
-	}
-
-	user := loginQuery.Result
-	loginUserQuery := models.LoginUserQuery{
+	authQuery := models.LoginUserQuery{
 		Username: username,
 		Password: password,
-		User:     user,
 	}
-	if err := bus.Dispatch(&loginUserQuery); err != nil {
+	if err := bus.Dispatch(&authQuery); err != nil {
 		ctx.Logger.Debug(
 			"Failed to authorize the user",
 			"username", username,
@@ -190,6 +177,8 @@ func initContextWithBasicAuth(ctx *models.ReqContext, orgId int64) bool {
 		return true
 	}
 
+	user := authQuery.User
+
 	query := models.GetSignedInUserQuery{UserId: user.Id, OrgId: orgId}
 	if err := bus.Dispatch(&query); err != nil {
 		ctx.Logger.Error(