General refactoring so the code is testable.
Extracted functions into a separated script file.
Initial tests.
Making better use of Given-When-Then.
Added a test for Get-NewestFilePath
Mocking looks good.
Added test for Update-AssemblyInfo and better descriptions.
Fixing Update-AssemblyInfo.

Added test for Get-EnvironmentVariableOrDefault.
Added tests for:

- Get-Version.
- The last buildCommands.
Large refactoring
Tests for Get-Extensions.
Test for Invoke-Extensions.
Extracted running nunit tests to Invoke-NunitTests.
Refactoring.
Test for Get-NugetBinary.
Fixed some issues with powershell "flattening" arrays returned by functions when there was one element.

Test for Get-Tests.
Changes to build.ps1 so it runs pester tests and returns greater than 0 on failure.
Fixes after running this for the first time.
Fixing Invoke-Extensions "when calling it with null".
Added Clean-Characters  so ps commands don't fail for some commands.