gdb/testsuite: Remove duplicates from gdb.base/pointers.exp

When I run the testsuite, I have :

    Running .../gdb/testsuite/gdb.base/pointers.exp ...
    DUPLICATE: gdb.base/pointers.exp: pointer assignment

Fix by placing the sections with duplication in with_test_prefix blocks.
This removes the duplication and gives a better organization the file.

Tested on x86_64-linux.
Co-Authored-By: Pedro Alves <pedro@palves.net>