BSC_Tests: fix a race condition in TC_imm_ass_pre_[ts,chan]_ack

These test cases sporadically fail on Jenkins and 9/10 times on my
machine.  The problem appears to be that we are sending both the
RSL CHANnel ReQuireD and the VTY command simultaneously, and the
later may be handled earlier than the former by osmo-bsc.

Ensure that the RSL message is handled first by sending the VTY
command after we have received the RSL CHANnel ACTivation message.

Change-Id: I38cd31041741b69eb15098a089b4d4b6b918ffd4
1 file changed