Fix global Terraform

[WhiteTomX]

Description

The global switch worked only when the module was not loaded as terraform autoloaded the module. Not the Module Invoke-WTerraform falls back to terraform.exe in Path.
Furthermore Set-WTerraform -Global now reloads the path

Related Issue

fix #18

Motivation and Context

How Has This Been Tested?

Added Test for global fallback and path reload.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.