test/common: suppress waitforlisten output on BSD

We currently use:

`./rpc.py 1>&2 2>/dev/null`

And apparently our stdout redirection gets treated as
a python argument, which is not what we intended. This
caused waitforlisten function to print some unnecessary
messages.

To fix, we could either wrap python with extra brackets
`$(./rpc.py) 1>&2 2>/dev/null` or use &>/dev/null. We
go with the latter.

Fixes #616

Change-Id: I49d2380392435447b7b93259055c1d02c4b9c157
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/442262


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>