Skip dev mode windows when choosing an existing window for an open action

[smashwilson]

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Identify the Bug

Fixes #19223, and an adjacent fault related to the same thing happening with benchmark windows.

Description of the Change

Check isSpec on candidate windows when locating an existing window to open paths in.

Alternate Designs

Rather than filtering out spec windows, I thought about flagging editor windows and adding a positive check instead. That way we'd be insulated against the addition of other new window types that shouldn't receive paths. This felt less invasive, though, and adding new window types is very rare.

Possible Drawbacks

N/A

Verification Process

• Add a unit test and get it to pass.
• Open an editor window and the spec runner in a dev build of Atom. Focus the spec runner, then run an atom a/1.md command on the command line. The non-spec window should be focused and the path added to it.

Release Notes

• Test and benchmark runner windows are no longer considered for opening paths.