🩹 [Patch]: Add tests for argument completions

### Describe the change

An example on how to test argument completions:

```pwsh
$command = 'Get-GitHubContext -Context '
$cc = [System.Management.Automation.CommandCompletion]::CompleteInput($command, $command.Length, $null)
$cc.CompletionMatches.CompletionText | Should -Contain 'github.com/MariusStorhaug'
```