[LIT] Tests take entire timeout duration after failing when exception is thrown in internal shell

[ayylol]

When using the internal lit shell, if a test throws an exception and a timeout is set the test will take the entire duration of the timeout before finishing. This can happen for example if we try to execute a non-existent file. The tests hang here, at the pool.join() call.

llvm-project/llvm/utils/lit/lit/run.py

Lines 87 to 93 in 1997053

	try:
	self._wait_for(async_results, deadline)
	except:
	pool.terminate()
	raise
	finally:
	pool.join()

This is related to this:
https://stackoverflow.com/questions/15314189/python-multiprocessing-pool-hangs-at-join

To Reproduce

1. Add a test that will try to execute a non-existent file:

; llvm-project/llvm/test/hang.ll
; RUN: non-existent

2. Change the test suite's configuration to use the internal shell. (Remove the not from this line)
   

   llvm-project/llvm/test/lit.cfg.py

   Line 21 in c1efd8b

   config.test_format = lit.formats.ShTest(not llvm_config.use_lit_shell)

3. Run the test with a time out set:

llvm-lit llvm/test/hang.ll -a --timeout=10

The test should fail immediately, but the execution of lit will not conclude until the full timeout elapses.

If instead we use the external shell, or not set a timeout then the execution of lit will conclude immediately after the test fails.