Powershell: add missing include directive

The missing `Import-Module` was likely masked by the non-isolated
nature of Concourse windows workers.

Author: aramprice

stembuild/stemcell-automation/AutomationHelpers.Tests.ps1

@@ -4,6 +4,7 @@
 BeforeAll {
     Import-Module ../modules/BOSH.SSH
     Import-Module ../modules/BOSH.Utils
+    Import-Module ../modules/BOSH.CFCell
     Import-Module ../modules/BOSH.Agent
     Import-Module ./AutomationHelpers.psm1