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

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.mi/mi-nsthrexec.exp ...
    DUPLICATE: gdb.mi/mi-nsthrexec.exp: breakpoint at main

Fix by adjusting the duplicated test name.

Tested on x86_64-linux.