Move copy_bitwise unittests to own unittest file

Move copy_bitwise unittests to own unittest file

Now that copy_bitwise has been made public, and considering that
its implementation could move to a different file again in the future,
this patch moves its unittest to its own file in gdb/unittests.

gdb/ChangeLog:

        * unittests/copy_bitwise-selftests.c: New file.
        * utils.c (selftests::bits_to_str, selftests::check_copy_bitwise)
        (selftests::copy_bitwise_tests): Delete, moving this code to
        unittests/copy_bitwise-selftests.c instead.
        (_initialize_utils): Do not register copy_bitwise tests.
        * Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
        unittests/copy_bitwise-selftests.c.

Tested on x86_64-linux using the official testsuite, but also by
verifying that "maintenance selftests" still runs the copy_bitwise
tests.