test: add Pester tests for Initialize-PSBuild

[nthbotast]

Closes #95

Summary

Adds focused Pester tests for Initialize-PSBuild as requested.

Covered scenarios

• ModuleOutDir is built correctly when OutDir is relative.
• ModuleOutDir is built correctly when OutDir already starts with BHProjectPath (case-insensitive path behavior).
• Set-BuildEnvironment is called with the expected BuildOutput.
• UseBuildHelpers gate behavior for environment variable output.

Notes

• Set-BuildEnvironment is mocked to avoid side effects.
• This is scoped to unit behavior for Tests: Initialize-PSBuild #95.