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

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/ptype.exp ...
    DUPLICATE: gdb.base/ptype.exp: ptype the_highest
    DUPLICATE: gdb.base/ptype.exp: list intfoo
    DUPLICATE: gdb.base/ptype.exp: list charfoo

Fix by adjusting the offending test names.

Tested on x86_64-linux.