test: Cover Initialize-PSBuild behavior

[pttydou]

Summary

• Add Pester 6 unit coverage for relative and project-rooted OutDir resolution
• Verify ModuleOutDir and the mocked Set-BuildEnvironment call receive the expected BuildOutput with -Force
• Restore BuildHelpers environment variables after each test

Closes #95

Test Plan

• Targeted Pester 6.0.0 test on PowerShell 7.6.4 — 2 passed, 0 failed
• Targeted Pester 6.0.0 test on Windows PowerShell 5.1.26100.4652 — 2 passed, 0 failed
• Focused Invoke-ScriptAnalyzer — no diagnostics
• Full suite — 435 passed, 2 failed, 2 skipped; both failures reproduce on unmodified main (missing MAML XML in tests/build.tests.ps1)

Breaking Changes

None.