Tests: Build-PSBuildUpdatableHelp

[HeyItsGilbert]

Add Pester 5 integration tests for Build-PSBuildUpdatableHelp.

Scenarios to cover:

• Updatable help cabinet files are produced correctly
• Requires platyPS + MakeCab (Windows-only — skip test on Linux runners)
• Depends on shared fixture module (see Tests: Create shared fixture module #97

Parent issue: #17