Generalize `PYTEST_CURRENT_TEST` for multiple threads

[Liam-DeVoe]

Part of #13768. See also discussion in #13837.

The PYTEST_CURRENT_TEST envvar stores the node id and phase of the current test. When multiple threads are running, we'll need to generalize this, so that each thread shows the current test being run.

Based on @RonnyPfannschmidt's comments in #13837, one option is that pytest makes it a requirement that whoever spawns the threads* sets each thread name to a scheme recognized by pytest, like pytest-thread-{n}. Pytest would check threading.current_thread().name in runtestprotocol, and then if it matches pytest-thread-{n}, use n to set the PYTEST_CURRENT_TEST_THREAD_{n} envvar.

* this responsibility might fall to pytest, or to the user, depending on TBD details about how a user invokes pytest multithreading. Eg, it might be invoked with a high-level + pytest-managed --threads arg, or a user-managed override of pytest_runtestloop and spawning a thread pool, or an alternative mechanism.

Main thread considerations

@RonnyPfannschmidt mentions that PYTEST_CURRENT_TEST should be reserved for the main thread. If we check this with threading.main_thread(), then we'll mistake cases where pytest is run inside of a thread:

import threading
import pytest

thread = threading.Thread(target=pytest.main)
thread.start()

We could:

1. Accept this as a possibility and don't worry about it
2. Treat the threading.current_thread() when pytest.main is called as "this pytest session's main thread", even if it's not truly the main thread
3. Forgo PYTEST_CURRENT_TEST entirely under threading, and just use PYTEST_CURRENT_TEST_THREAD_{n}

[RonnyPfannschmidt]

Let's set the constraint that only a single pytest session is valid per process

Then the threads under pytest controll can pick the other ids

[The-Compiler]

@RonnyPfannschmidt mentions that PYTEST_CURRENT_TEST should be reserved for the main thread.

That seems to me like it would break too many things. Reviewing how the env var is used it seems to me like the majority of use cases is checking whether code is run "under testing", and if so, change its behavior somehow (whether that's a good idea is a different discussion).

If PYTEST_CURRENT_TEST is not set in other threads, it means code like such will break (probably in subtle ways) depending on which thread it's called in.

Also, our documented use case is for debugging, when pytest hangs. I feel like having many different environment variables would make this much harder.

Since we clearly say the value is only intended for human consumption, could we not just set it (in all threads) to something like (separators tbd, using a newline here for readability):

1: test_threads::test_one (call)
2: test_threads::test_two (call)

or so?