In trans_free(), call subscr_conn_release_when_unused(), so that we are sure to
clean up after the last transaction is done.
This fixes an error where a conn lingered after a CC failure, because that code
path forgot to trigger cleanup.
Rationale: so far we were triggering the release check after each DTAP dispatch
(compl_l3 and "normal" DTAP), which is sufficient for properly closed
transactions. We also need a check for when a timeout clears an erratic trans.
Adjust test expectation of test_call_mo_to_unknown_timeout to show that the
error is now fixed.
msc_vlr_test_reject_concurrency now sees an additional release checking event
when the SMS transaction is done, which is expected and does not affect the
test otherwise.
Related: OS#2779
Change-Id: I46ff2e9b09b67e4e0d79cccf8c04936f17281fcb
d6a769b51c