tests: Disable ipmi-bt-test

tests: Disable ipmi-bt-test

The ipmi-bt-test fails intermittently, especially on the NetBSD VM.
The frequency of this failure has recently gone up sharply to the
point that I'm having to retry the NetBSD build multiple times
to get a pass when merging pull requests.

Disable the test until we can figure out why it's failing.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190118185402.3065-1-peter.maydell@linaro.org