The tag is present in Lintian version 2.104.318. That is the most recent version we know about.
There is an XS-Testsuite field in the
debian/control file. As
of dpkg 1.17.10, the XS- prefix is no longer necessary. dpkg now
recognizes this field and handles it correctly. As of dpkg 1.17.11 the
field is automatically added by dpkg-source with the value "autopkgtest" if
there is a non-empty
debian/tests/control file present. Consider
either removing the XS- prefix for this field or removing the field
altogether if it contains just the "autopkgtest" value.