Extract test pattern to helper
Extract test pattern to helper

An existing test in diskfile established an prior art for a pattern to
create a diskfile with a different suffix - I'd like to make use of it
in new tests in multiple unrelated change sets.

Also add a test to demonstrate some existing robustness and prevent
regression.

Author: Pavel Kvasnička <54e56da5c5b238d9db626dccb26c750f53fc72c3@firma.seznam.cz>
Co-Author: Alistair Coles <3e53127f3ad0d91a1ed4651b133d989b076d9d27@hpe.com>
Related-Change-Id: I64cadb1a3feb4d819d545137eecfc295389794f0
Change-Id: I3a661fae5c7cfeb2dbcdb7f46941f55244d0b9ad

clayg/swift,nadeemsyed/swift,notmyname/swift,psachin/swift,openstack/swift,swiftstack/swift,tipabu/swift,redbo/swift,psachin/swift,nadeemsyed/swift,tipabu/swift,clayg/swift,nadeemsyed/swift,matthewoliver/swift,psachin/swift,smerritt/swift,tipabu/swift,smerritt/swift,matthewoliver/swift,psachin/swift,openstack/swift,smerritt/swift,matthewoliver/swift,notmyname/swift,tipabu/swift,notmyname/swift,matthewoliver/swift,redbo/swift,openstack/swift,swiftstack/swift,nadeemsyed/swift,clayg/swift,smerritt/swift,clayg/swift,swiftstack/swift,openstack/swift,notmyname/swift