long running (or stuck) SUT shutdown can deadlock systest_watchdog

[hyperthunk]

This is probably a duplicate of issue #57. The root cause appears to be an infinitely hanging SUT shutdown. What happens is roughly thus:

1. a ct_hook calls systest_watchdog:force_stop/1
2. the watchdog calls systest_sut:stop/1
3. the sut calls systest_cleaner:kill_wait/2
4. the cleaner blocks

This situation leads to the watchdog becoming permanently blocked, which in turn prevents future SUTs from starting, and we end up with gen_server:call/2 timeouts everywhere (or subsequent testcase time traps).

All of this is very unclear to the test author and operator, so we need to tidy it all up somehow.

[hyperthunk]

The fix for this is going to be a configurable timeout for shutdowns. IF this timeout is triggered, then we're going to skip all further testing and make the operator fix things before continuing.

[hyperthunk]

Marking as WONTFIX as we are going to address this via issue #75 and co.