fix new test for pwsh 5.1

Author: santisq

tests/PSParallelPipeline.tests.ps1

@@ -245,7 +245,14 @@ Describe PSParallelPipeline {
                     AddScript('0..10 | Invoke-Parallel { Start-Sleep 1; $_ }')
                 $async = $ps.BeginInvoke()
                 Start-Sleep 1
-                $async = $ps.BeginStop($ps.EndStop, $null)
+
+                if ($IsCoreCLR) {
+                    $async = $ps.BeginStop($ps.EndStop, $null)
+                }
+                else {
+                    $ps.Stop()
+                }
+
                 while (-not $async.AsyncWaitHandle.WaitOne(200)) { }
                 $timer.Stop()
                 $timer.Elapsed | Should -BeLessOrEqual ([timespan]::FromSeconds(1.5))