There's not enough tests to verify such cases:
 - b.split()[0] is not b
 - b.rsplit()[0] is not b
 - b.splitlines()[0] is not b
 - b.partition('.')[0] is not b
 - b.rpartition('.')[0] is not b
 - (other ?)

However similar tests exist in Lib/test/test_bytes:
 * ByteArrayTest.test_copied()
   -> some tests for ('replace', 'translate')
 * ByteArrayTest.test_partition_bytearray_doesnt_share_nullstring
   -> some tests for ('partition', 'rpartition')
 * BytearrayPEP3137Test.test_returns_new_copy()
   -> some tests for ('zfill', 'rjust', 'ljust', 'center')