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

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/unload.exp ...
    DUPLICATE: gdb.base/unload.exp: continuing to unloaded libfile

Fix by adjusting the test name.

Tested on x86_64-linux.
1 file changed