remove test_multiprocess_api

This test suite should be removed for the following reasons.

1) it's not a unit test

2) it's failing randomly, a lot, and depends on timing of the host for
process start and signal handling which are provably not predicable on
slower machines

3) it's triggering testr bugs in the ways that it fails which means we
don't actually get any subunit in the fail scenario, which makes it
actually impossible to move forward on fixes.

Change-Id: Ieb7c02115c12cd9f6c9fa67691a643aed7632784
Closes-Bug: #1357578