Add autoload for test modules (#3342)

Related #3238

1. Add autoload for test modules
2. Move TestHostCS.psm1 to 'test\tools\Modules\' folder
3. Remove explicit load TestHostCS.psm1 from test files

test/powershell/Modules/Microsoft.PowerShell.Security/GetCredential.Tests.ps1

@@ -1,6 +1,3 @@
-$hostmodule = Join-Path $PSScriptRoot "../../Common/TestHostCS.psm1"
-import-module $hostmodule -ErrorAction SilentlyContinue
-
 Describe "Get-Credential Test" -tag "CI" {
     BeforeAll {
         $th = New-TestHost