Added running_in_another_thread()

[radarhere]

Possible way to help #8492. Alternative to #8501

I've created a Tests/helper.py function called running_in_another_thread() that uses a global variable and inspect to determine if a test has already started running in another thread, so that we can return early if we would only like the test to run in one thread at a time.

This is used here in two cases

1. Running the tests in multiple threads fails because of thread-unsafe Python code #8492 has found that running a test that calls register_handler at the beginning, and removes the handler at the end, fails when the test is run across multiple threads. My thinking is that if it ok for register_handler to operate across threads, and that it doesn't need to register multiple handlers in parallel. So this PR returns early from the relevant test if it is already running in another thread.
2. Found concurrency errors in Pillow tonybaloney/pytest-freethreaded#10 finds an error when running
   

   Pillow/Tests/test_core_resources.py

   Lines 50 to 54 in 16372dd

   	def test_set_alignment(self) -> None:
   	for i in [1, 2, 4, 8, 16, 32]:
   	Image.core.set_alignment(i)
   	alignment = Image.core.get_alignment()
   	assert alignment == i

   
   across multiple threads. I again think that Image.core.set_alignment doesn't need to be run across multiple threads simultaneously, so this PR returns early.

[lysnikolaou]

I think that this introduces another possible race, where two or more threads all think that the test is not currently running elsewhere and add themselves to the list.

[lysnikolaou]

If I were to choose between this and #8501, I'd probably go with #8501. This introduces another piece of global state that we probably can avoid.