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

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.mi/mi-nonstop-exit.exp ...
    DUPLICATE: gdb.mi/mi-nonstop-exit.exp: breakpoint at main
    DUPLICATE: gdb.mi/mi-nonstop-exit.exp: mi runto main

This test runs the same sequence of operations twice.  Refactor the code
by running both of those sequences within a foreach_with_prefix block to
ensure that the commands have unique test names.

Tested on x86_64-linux.