Add GuiNewWindow Test Coverage

[jgehrig]

We should have UnitTest coverage for this feature.

Consider covering the following:

• Does a new window actually open?
• Are --server arguments forwarded correctly?
• Are --nvim arguments forwarded correctly?
• Out of order close, close first window?
• Exit Code

It would be nice to isolate the tests into a new test file. tst_newwindow?

If we must, we can add these to tst_shell. However, you'll likely hit merge conflicts with some upcoming work.