test.py: don't fail if test's exit code is not 0 on --help
test.py: don't fail if test's exit code is not 0 on --help

test.py invokes all test executables once with --help to determine
whether it needs a -- to seperate scylla args or not. For this check it
doesn't matter what exit code the test exits with, so don't fail if it's
not 0.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <d05be7c3819349e3b22b6249bb83fbf9269d14cb.1517314408.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla