gdb/testsuite: Remove duplicates from gdb.mi/mi-nonstop.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.mi/mi-nonstop.exp ...
    DUPLICATE: gdb.mi/mi-nonstop.exp: check varobj, w1, 1
    DUPLICATE: gdb.mi/mi-nonstop.exp: stacktrace of stopped thread

Fix by adjusting the problematic test names.

Tested on x86_64-linux.