lapd: Extend the test to check if lapdm_channel_exit is idempotent

I have a crash with an older libosmocore and the sysmobts. I
am not sure how we can have a release for a channel where the
lapdm_channel has not gone through lapdm_channel_init. But to
prevent the crash we can promise lapdm_channel_exit to be idempotent.

It has been idempotent since f5a079f739c57d8be7c59149fd45475c402a45fc
1 file changed