Missing dep8 tests

As of July 28, 2021, this source package did not contain dep8 tests in
the current development release of Ubuntu, named Impish. This was
determined by running `pull-lp-source bind9-libs impish` and then
checking for the existence of 'debian/tests/' and
'debian/tests/control'.

Test automation is essential to higher levels of quality and confidence
in updates to packages. dep8 tests [1] specify how automatic testing can
be integrated into packages and then run by package maintainers before
new uploads.

This defect is to report the absence of these tests and to report the
opportunity as a potential item for development by both new and
experienced contributors.

[1] http://packaging.ubuntu.com/html/auto-pkg-test.html