Tests: Build-PSBuildModule

[HeyItsGilbert]

Add Pester 6 integration tests for Build-PSBuildModule.

Scenarios to cover:

• Source files are staged correctly to the output directory
• Monolithic PSM1 compilation works when enabled
• Depends on shared fixture module (see Tests: Create shared fixture module #97)

Target Pester 6.0.0 with classic Should syntax — see notes on #17. Parent issue: #17