BSC_Tests: fix race conditions in TC_imm_ass_pre_{ts,chan}_ack_dyn_ts

The TC_imm_ass_pre_chan_ack_dyn_ts sporadically fails on Jenkins.
Similar to [1], this is happening because the VTY command setting
the 'immediate-assignment' strategy back to 'post-chan-ack' is
being sent/processed earlier than the CHANnel REQ message.

TC_imm_ass_pre_ts_ack_dyn_ts is likely affected too, also fix it.

Change-Id: I1e38142d29d0fa2946a858eac13319aa05b42aa3
Related: [1] I38cd31041741b69eb15098a089b4d4b6b918ffd4
1 file changed