vty-tests: Re-introduce the check on the help
With the support of regex within vty tests,
the help message can be checked again.
Depends-on: Iadcd7a8c3677548a6405e098fe53d0614ef2012c (osmo-python-tests)
Change-Id: I3fad268b7a8925f378f6331fcab073637bd80e08
