sandbox: start sandbox in a pseudoterminal

[Bo98]

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This disconnects the sandbox from directly accessing the parent terminal.

Due to the wide range of formulae we have and how some of the tests we have can be pretty picky on the terminal, particularly for those that do things like manually setting TERM, changing the winsize or doing some fancy things in expect, I can't give 100% assurances this will be regression free but it does seem the work on a couple that I tried. It's very much possible nothing will break and I'm being overly cautious - there's just too much variation for me to guarantee anything.

In the event someone feels the need to revert this quickly, but the only thing breaking is (multiple) tests, I suggest a partial revert by adding a kwarg to conditionally disable this for tests (but not builds/postinstall) should time allow.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Let's try it out.