tests/mboxviews: test for unchanged postscript

tests/mboxviews: test for unchanged postscript

Currently a patch containing postscript is always modified in patch_to_mbox()
compared to the input patch.

A comment containing 'some comment\n---\n some/file | 1 +' will be changed to
'some comment\n\n---\nsome/file | 1 +\n' which is annoying.

This patch adds a test to detect that, a follow up patch will fix the error
then.

Signed-off-by: Andreas Bießmann <andreas@biessmann.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>