t: Add optional "Test-Depends" field

Add support for the (optional) field, "Test-Depends".  If present, the
dependencies listed in the field will be checked.  If the dependency
field is unsatisfiable, the test is skipped.

This is useful for skipping tests that cannot be done in (e.g.)
stable, where some packages are missing or too old.

Signed-off-by: Niels Thykier <niels@thykier.net>
t: Move #DEBHELPER# token in a test

This makes the test immune to changes to the "default" snippets
inserted by debhelper.  Notably, debhelper in Wheezy and Jessie
produces a different output, which would cause the test to fail
when run in Wheezy.

Signed-off-by: Niels Thykier <niels@thykier.net>