Fail tests that leak pipes

[sunshowers]

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