Whitelist watchdog threads in test thread leak checker

The watchdog threads check their status every 10 seconds which means
they can only exit as quickly as once every 10 seconds. This results in
the threads appearing to be leaked by tests but they should go away in
no more than 10 seconds.

Note that we don't reduce this poll time as there could be many watchdog
threads running during normal zuul execution and typically 10 second
resolution there is plenty. We just have to accomodate that in our
tests.

Change-Id: If3cf86c4af0b2fbcaf51a233c766c75749ca1d1d
2 files changed