Shorten TestDrive directory name (#2239)

* 2238  Shorten TestDrive directory name.

Pester currently uses a full GUID as its temp directory name, which uses 36 characters. This contributes to test errors
when paths used in tests exceed the Windows max path of 260 characters.

The `[IO.Path]::GetTempFileName()` only uses 4 characters for the unique/random part of its filename. Follow the
same pattern in `New-RandomTempDirectory` by only using the 4 characters. A GUID, when converted to a string, only uses
hex characters: 0-9 and a-f, which gives us 65,536 unique possible values. Instead, lets use
`[IO.Path]::GetRandomFileName()`. It generates a name that uses all letters of the alphabet and the numbers 0-9, which
gives us 36^4 = 1,679,616 possible values.

* shorter name in registry

* Pester_ prefix

---------

Co-authored-by: Jakub Jareš <me@jakubjares.com>

src/functions/TestDrive.ps1

@@ -125,7 +125,8 @@ function Clear-TestDrive {
 function New-RandomTempDirectory {
     do {
         $tempPath = Get-TempDirectory
-        $Path = [IO.Path]::Combine($tempPath, ([Guid]::NewGuid()));
+        $dirName = 'Pester_' + [IO.Path]::GetRandomFileName().Substring(0, 4)
+        $Path = [IO.Path]::Combine($tempPath, $dirName)
     } until (-not [IO.Directory]::Exists($Path))
 
     [IO.Directory]::CreateDirectory($Path).FullName

src/functions/TestRegistry.ps1

@@ -73,7 +73,7 @@ function Get-TestRegistryChildItem ([string]$TestRegistryPath) {
 function New-RandomTempRegistry {
     do {
         $tempPath = Get-TempRegistry
-        $Path = & $SafeCommands['Join-Path'] -Path $tempPath -ChildPath ([Guid]::NewGuid())
+        $Path = & $SafeCommands['Join-Path'] -Path $tempPath -ChildPath ([IO.Path]::GetRandomFileName().Substring(0, 4))
     } until (-not (& $SafeCommands['Test-Path'] -Path $Path -PathType Container))
 
     try {