gdb/testsuite: Remove duplicates from gdb.base/define-prefix.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/define-prefix.exp ...
    DUPLICATE: gdb.base/define-prefix.exp: define user command: ghi-prefix-cmd

Fix by adjusting test names.

Tested on x86_64-linux.
1 file changed