For out-of-tree code that may set up multiple VUTs in a single process,
we don't want the VUT that initialized a PID file to remove it if it
finishes before other VUTs.

A global pseudo-VUT is introduced to keep track of the PID file and
the error callback in order to use them in the atexit(3) callback.

Original test case by Geoff, slightly modified.

Closes #2651