py3: use absolute_import in test-bdiff.py

tests/test-bdiff.py

@@ -1,5 +1,9 b''
+from __future__ import absolute_import
 import struct
-from mercurial import bdiff, mpatch
+from mercurial import (
+    bdiff,
+    mpatch,
+)
 
 def test1(a, b):
     d = bdiff.bdiff(a, b)

tests/test-check-py3-compat.t

@@ -53,7 +53,6 b''
   tests/svn-safe-append.py not using absolute_import
   tests/svnxml.py not using absolute_import
   tests/test-atomictempfile.py not using absolute_import
-  tests/test-bdiff.py not using absolute_import
   tests/test-bdiff.py requires print_function
   tests/test-context.py not using absolute_import
   tests/test-context.py requires print_function