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

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/readline.exp ...
    DUPLICATE: gdb.base/readline.exp: Simple operate-and-get-next - final prompt

Fix by adjusting the prefix given to the second 'simple' call to
operate_and_get_next.

Tested on x86_64-linux.