help tests: add test for --config output

Add a missing test for checking what the --config output added in ac68a93fd2 (help: add --config to list all available config, 2018-05-26) looks like. We should not be emitting anything except config variables and the brief usage information at the end here. The second test regexp here might not match three-level variables in general, as their second level could contain ".", but in this case we're always emitting what we extract from the documentation, so it's all strings like: foo.<name>.bar If we did introduce something like variable example content here we'd like this to break, since we'd then be likely to break the git-completion.bash. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>