tests/test-dirstate-backup.t

tests: conditionalize tests for various repo features

Working my down the long tail of test failures due to the simple
store.

We're now down to 38 failures with the simple store.

Differential Revision: https://phab.mercurial-scm.org/D3096

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [255]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate