tests/qtest: Run ipmi-bt-test only if CONFIG_IPMI_EXTERN is set

The ipmi-bt-test uses "-device ipmi-bmc-extern", thus it should
only be run if this device has been enabled in the configuration.

Message-Id: <20230524081024.1619273-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>