12061: Explicitly join actor thread after stopping.

12061: Explicitly join actor thread after stopping.

Fixes the "nonfatal" test case the same way the "fatal" test case was
fixed in 6706c6d46c6cfb080371b147b60fcaa191bd55a6.

Since the nonfatal case just tests that kill_mock did *not* get
called, the race condition used to make the test ineffectual
occasionally, instead of failing occasionally.

The race condition became visible only after the status reporting test
was added.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>