Update Test-PSModule.ps1

Don't always install Pester since it's now a required module.

Author: thisjustin816

src/public/Test-PSModule.ps1

@@ -1,4 +1,4 @@
-<#
+<#
 .SYNOPSIS
 Tests a PowerShell module using Pester.
 
@@ -33,8 +33,6 @@ function Test-PSModule {
         [String[]]$Tag
     )
 
-    Install-Module -Name Pester -Force -AllowClobber -SkipPublisherCheck
-
     Get-Module -Name $Name -All | Remove-Module -Force -ErrorAction SilentlyContinue
     $config = New-PesterConfiguration @{
         Run          = @{