tests/test-backwards-remove.t

test-manifest: make manifesttest a base class that is extended

The implementation of the testmanifest test case is moved to a new base class,
which is then extended to make the testmanifest. And instead of testmanifest,
the subclass is named testmanifestdict because, well, that's what it's testing.

This refactoring makes it possible to create alternate versions of what was
formerly testmanifest, improving test coverage of different manifestdict
implementations.

  $ 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