[trivial] Remove src/translate/{tests,regression-tests}.

We discussed this in Discord, and there were no objections.
Copying the relevant message:

While working on some translator code, we stumbled over the directories
src/translate/tests and src/translate/regression-tests/.

We haven't touched tests since 2015 except for global changes like
Python 2 => Python 3.

We haven't touched regression-tests since 2013.

As far as I remember, these predate the tests we actually use (and which
live in a different part of the repository), and they don't satisfy the
requirements you'd typically want of tests (not all are meant to be
automatic tests in the usual way; the ones that are don't necessarily
pass, even though there isn't a bug; the one that perhaps passes doesn't
test something we still consider important, such as direct executability
of certain translator source files).

I suggest we delete these directories. If someone is keen on the
functionality they were intended for, I think we would need to invest
some work to make them useful. But I'm not convinced this is good value
for time.