tests: Add 'series' tests
tests: Add 'series' tests

We need to start adding tests to make sure, if nothing else, that we
don't regress. Start with the 'series' commands.

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

getpatchwork/git-pw,stephenfin/git-pw