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.