The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The dsc file sets Testsuite to an unrecognised value. This field is most probably copied by dpkg-source from Testsuite in debian/control.
Please refer to https://salsa.debian.org/ci-team/autopkgtest/tree/master/doc/README.package-tests.rst for details.