fix(files): remember explorer's tabpage when file is opened in new tab

[abeldekat]

• I have read CONTRIBUTING.md
• I have read CODE_OF_CONDUCT.md

Resolve #2388

[echasnovski]

Thanks for the PR!

This looks like the correct solution indeed. Would you mind please adding two tests:

• One for close() returning nil and not changing the tabpage in the issue's scenario.
• One for open() opening the explorer in the current tabpage in the issue's scenario.

[echasnovski]

This should now be part of the latest main. I've adjusted tests: replaced screenshot testing for correct opening with counting floating windows, made better coverage for use_latest argument of open(), and made both tests a bit more consistent with each other.

Thanks again for the PR!

[abeldekat]

Great! Thanks.