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.