check for, install or update of Pester (#187)

* check for, install or update of Pester

* Linux doesn't (yet) have Pester installed by default

Author: axtens

bin/run-tests.ps1

@@ -1,3 +1,8 @@
+# Already present in Windows install
+if (-Not (Get-Module -ListAvailable -Name Pester)) {
+    Install-Module -Name Pester -Force -SkipPublisherCheck
+}
+
 $tmpRoot = [System.IO.Path]::GetTempPath()
 $exercises = Get-ChildItem -Path $PSScriptRoot/../exercises/practice -Directory
 $failed = 0