Add test to ensure that "stg clean" preserves conflicting patches

If "stg push" fails, the subsequent "stg clean" will remove the patch
that could not be applied. I think it's wrong. Especially when doing
"stg pull", it can happen that I want to run "stg clean" to get rid of
the patches applied upstream so I can concentrate on the conflict.
Instead, the conflicting patch is removed too.

The test added by this patch should pass once the bug is fixed.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Karl Hasselström <kha@treskal.com>