tests/test-backwards-remove.t

packaging: add Cosmic and Disco, remove Trusty and Artful

- Trusty was publicly supported until 2019-04-30
- Artful was publicly supported until 2018-07-19
- Cosmic was publicly supported until 2019-07-18
- Disco will be publicly supported until 2020-01

Cosmic is officially out-of-date, but since it still may be in use, and because
we didn't add it when it first came out, I think it would be nice to support it
until the next time somebody decides to update this list of Ubuntu releases.

Differential Revision: https://phab.mercurial-scm.org/D6761

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls
  a