fix TestRepoDirCheck::test_null_bytes

Replace a 'utf_16_be'-decodable byte string with another, which is
non-decodable.

Signed-off-by: Anna (cybertailor) Vyalkova <cyber+gentoo@sysrq.in>
Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>

Author: @CyberTailor

tests/checks/test_repo.py

@@ -64,7 +64,7 @@ def test_ignored_root_dirs(self):
     def test_null_bytes(self):
         check = self.mk_check()
         with open(pjoin(self.repo.location, "foo"), "wb") as f:
-            f.write(b"foo\x00\xffbar")
+            f.write(b"foo\x00\xff\xffbar")
         r = self.assertReport(check, [])
         assert isinstance(r, repo.BinaryFile)
         assert r.path == "foo"