quilt-gbp test: Set DGIT_TEST_TOLERATE_UNPATCHED_CLEAN

In quilt-gbp we want to test whether dgit detects that quilt fixups
cannot be linear because the user has failed to pass an appropriate
quilt option in a patches-unapplied tree.

build_source() cleans before applying patches, and the package's clean
target might error out because patches are not applied.  This prevents
dgit from attempting quilt linearisation and thus determining that the
tree might be patches-unapplied.  So set an environment variable
telling the package's clean target not to do that.

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>