attempt to fix a race condition in BSC test's f_ts_dyn_mode_get

Add two helper functions which retry a VTY command until the
result matches a regular expression or a configurable timeout
expires.

Use these functions in BSC test's f_ts_dyn_mode_get, which has
seen sporadic failures due to a race condition during channel
reconfiguration, in order to hopefully close this race.

Change-Id: I308ddb06e440c165fe1e73fe2c1fb78be2e1d510
Related: OS#3690
2 files changed