dpkg-source: Generate Testsuite-Triggers field from test dependencies

Sometimes autopkgtests regress due to change in a package which is only
a test dependency (Depends: in debian/tests/control), not a build or
binary one. It is useful to trigger a test if such a test dependency
changes.

Record the union of all test dependency packages in a new
Testsuite-Triggers field in the .dsc, so that they will be recorded in
the Sources package index. Ignore versions and flatten OR dependencies
as they are not interesting for determining reverse test dependencies
and should not be (ab)used for replacing debian/tests/control parsing.

Closes: #779559
LP: #1491145


Based-on-patch-by: Martin Pitt <martin.pitt@ubuntu.com>
Signed-off-by: Guillem Jover <guillem@debian.org>