Add global setup/teardown hooks for test suites

[kikijiki]

It would be helpful to have support for global setup and teardown functions that run once before all tests begin and after all tests complete, respectively. This pattern is common in testing frameworks.

[sunshowers]

Sure, seems reasonable. I don't plan to work on this myself unless my employer or I need it, but if you'd like to contribute this feature I'd be happy to review it.

In #254 I converted the macro to use a tt-muncher, which should make this easier. I'd write this as optional setup and teardown arguments. Things to consider include:

• Should the list of tests/files to execute be provided to the setup function? If so, how, in a
  way that ensures forward compatibility?
• If there are multiple setup and teardown arguments, how should they be ordered?
• Should the functions be called if there are no tests? (I'd lean towards no, due to nextest compatibility -- setup and teardown will effectively be called once per test with nextest).
• For the teardown function, how should the statuses be provided?

If you can write up a design proposal, we can go over that before you get to implementing it.

[kikijiki]

I made a quick PR to illustrate what I had in mind.