Fail tests that leak pipes

It is possible to write a test that ends up leaking a pipe (e.g. creates a process but doesn't terminate it). Currently, the test runner hangs on encountering such a test.

Instead, we should figure out a way to:
* detect such a situation (a small amount of raciness between waiting on process exit and checking that the handles are closed is fine)
* mark such a test as failure with a `LEAK` message or similar

Start by locating the test runner logic that waits for process exit and checks pipe handles, then reproduce a test that creates a process without terminating it. Implement leak detection that prevents the runner from hanging and reports the affected test as a failure with a `LEAK` message; verify the normal process-cleanup path still works.

Written by the indexing model from the issue text.