test suite: dch: Always use tstunt/debchange

We are going to make this more sophisticated, so use it everywhere to
avoid duplication.

We need to soup up tstunt/debchange so that it does not depend on
DGIT_TEST_REAL_DEBCHANGE.  If it hasn't been put on the PATH by
t-tstunt, that isn't set, and we can just run debchange from PATH.

This makes faketime a test dependency for all the tests which don't
have NO-DEFAULT.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>