Update Windows PowerShell's Pester on CI for tests

It comes with 3.0.4 which is ancient and cannot run our Pester tests.

Author: andyleejordan

.vsts-ci/templates/ci-general.yml

@@ -14,6 +14,16 @@ steps:
     script: $PSVersionTable
     pwsh: ${{ parameters.pwsh }}
 
+- task: PowerShell@2
+  displayName: Update Pester
+  inputs:
+    targetType: inline
+    script: |
+      Install-Module -Name Pester -Force -Scope CurrentUser
+      Get-Module -ListAvailable -Name Pester
+    pwsh: false
+  condition: not(${{ parameters.pwsh }})
+
 - checkout: self
 
 # NOTE: We either checkout the Git repo for PowerShellEditorServices, or we