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

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>