tests:ipmi: Fix IPMI BT tests

tests:ipmi: Fix IPMI BT tests

The IPMI BT tests had a race condition, if it receive an IPMI command
to enable interrupt, it would write the message to enable interrupts
after it wrote the command response.  So the test code could
receive the command response and issue the next command before the
device handled the interrupt enable command, and thus no interrupt.

So send the message to enable interrupt before the command response.

Also add some sleeps to give qemu time to handle responses, there was
no delay before, and it could result in an invalid timeout.

And re-enable the tests, as hopefully they are fixed now.

Note that I was unable to reproduce this even with the instructions
Peter gave me, but hopefully this fixes the issue.

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>