Create regression suite for #312

[tony]

Effects issues like #312, and spin-off tickets like #326/#370 (focus), #364 ("no previous window)

See https://github.com/tmux-python/tmuxp/issues/364#issuecomment-377795942.

#312 needs a regression suite built around it.

It's necessary because 2.6 introduced an issue where the dimensions of unattached screens can't calculate layout info/switch layouts. get what I mean? That's why the hook was added.

The the thing is, the hook may need to be tailored to fit more specific instances so issues like this don't bubble up. Whether it's:

• the conditions under which the hook itself is created
• the hook's specificity to a target/global
• when it's added in the workspace creation process, or
• adding more conditions inside of the callback itself