[Ubuntu] Migrate PowerShell and Docker tests to Pester (#2317)

* PowerShell and Docker Pester tests

* no pester module

* import pester

* invoke docker with sudo

* remove native test

* add fix for az 1.0.0 module

* revert powershellget installation

* revert flag Force

* add $ProgressPreference = "SilentlyContinue"

images/linux/scripts/helpers/Tests.Helpers.psm1

@@ -16,6 +16,11 @@ function Invoke-PesterTests {
         throw "Unable to find test file '$TestFile' on '$testPath'."
     }
 
+    # Check that Pester module is imported
+    if (!(Get-Module "Pester")) {
+        Import-Module Pester
+    }
+    
     $configuration = [PesterConfiguration] @{
         Run = @{ Path = $testPath; PassThru = $true }
         Output = @{ Verbosity = "Detailed" }