Changed using azure test framework help docs. (#15797)

* removed set PSMODULEPATH command.

* Changed the logic of creating Az role assignment in TestFx-Task.psm1.

* changed testing help docs for set test envirmonent variables via commands.

* Restore tools/PSRepoStart.cmd.

* modified using-azure-test-framework.md.

* Update using-azure-test-framework.md

Co-authored-by: Dingmeng Xue <[email protected]>

Author: LucasYao93

documentation/testing-docs/using-azure-test-framework.md

@@ -20,6 +20,9 @@
 - [Environment Variable Reference](#supported-environment-in-test-framework)
 
 ## Getting Started
+-  Install the latest Az.Resources from the [PSGallery](https://www.powershellgallery.com/) into Windows PowerShell
+    - Run Windows PowerShell as administrator and execute following command
+        - `Install-Module -Name Az.Resources -Scope AllUsers -AllowClobber` 
 -  Double click `.\tools\PS-VSPrompt` shortcut
 	- This starts the VS Developer command prompt in PowerShell inside the `azure-powershell/tools` directory
 - Import the `Repo-Tasks` module that helps to perform basic repository tasks

tools/PSRepoStart.cmd

@@ -1,2 +1,4 @@
 @echo off
+SET currDir=%~dp0
+SET PSModulePath=%PSModulePath%;%currDir%\Modules
 "%VS140COMNTOOLS%VsDevCmd.bat"