Use Path.Combine for create directory (#6630)

Path.Combine will allow for fully qualified paths to override the combination. For example `Path.Combine("a","b","c:\test")` will resolve to `c:\test'. We have depended on such behavior in a few places like d66b5160f2/eng/pipelines/templates/steps/smoke-test-steps.yml (L99) so I'm reverting back to Path.Combine.

Co-authored-by: Wes Haggard <weshaggard@users.noreply.github.com>

eng/common/TestResources/New-TestResources.ps1

@@ -163,7 +163,7 @@ try {
     $root = $repositoryRoot = "$PSScriptRoot/../../.." | Resolve-Path
 
     if($ServiceDirectory) {
-        $root = "$repositoryRoot/sdk/$ServiceDirectory" | Resolve-Path
+        $root = [System.IO.Path]::Combine($repositoryRoot, "sdk", $ServiceDirectory) | Resolve-Path
     }
 
     if ($TestResourcesDirectory) {