tests: only run ipmi-bt-test if CONFIG_LINUX

tests: only run ipmi-bt-test if CONFIG_LINUX

This test has been unstable on NetBSD for awhile. It seems the
mechanism used to listen to a random port is a Linux-ism (although a
received wisdom Linux-ism rather than a well documented one). As
working around would add more hard to test complexity to the test I've
gone for the easier option of making it CONFIG_LINUX only.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Corey Minyard <cminyard@mvista.com>
Cc: Kamil Rytarowski <kamil@netbsd.org>