https://github.com/ncoghlan/cpython/pull/2/files provides a test case that reliably reproduces the problem for both synchronous and asynchronous context managers.

It's inspired by Nathaniel's proposal above, but relies on a modified version of sys.settrace that runs the trace function after every opcode, not just every time the line number changes or we jump backwards in the bytecode.

Issue 31344 is a separate issue to add that underlying capability so we can write this test case.