gitattributes: disable blank-at-eof errors for clar test expectations

The clar unit testing framework carries a couple of files that contain
expected output for its self-tests. Some of these files expectedly end
with a blank line at the end of the file, which Git would consider to be
a whitespace error by default.

Teach our gitattributes to ignore those errors.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

.gitattributes

@@ -17,3 +17,4 @@ CODE_OF_CONDUCT.md -whitespace
 /Documentation/gitk.adoc conflict-marker-size=32
 /Documentation/user-manual.adoc conflict-marker-size=32
 /t/t????-*.sh conflict-marker-size=32
+/t/unit-tests/clar/test/expected/* whitespace=-blank-at-eof