Tests: Create shared fixture module

[HeyItsGilbert]

Create a minimal skeleton module at tests/fixtures/TestModule/ (a bare .psm1 + .psd1) to be shared across all integration tests.

This is a prerequisite for the Build-PSBuildModule, Build-PSBuildMarkdown, Build-PSBuildMAMLHelp, Build-PSBuildUpdatableHelp, Test-PSBuildPester, and Publish-PSBuildModule sub-tasks.

Parent issue: #17