Simplify unit tests for `Assert()` and `Normalize()`

The unit tests for `Assert()` and `Normalize()` could be simplified by mocking the method `GetDesiredState` in the instance object.

```powershell
$script:mockInstance |
    Add-Member -Force -MemberType 'ScriptMethod' -Name 'GetDesiredState' -Value {
        # Optional return value
    } 
```