gdb/testsuite: adjust add-inferior test for native-extended-gdbserver board

gdb/testsuite: adjust add-inferior test for native-extended-gdbserver board

I see this failure:

    $ make check TESTS="gdb.base/with.exp" RUNTESTFLAGS="--target_board=native-extended-gdbserver"
    FAIL: gdb.base/with.exp: per-inferior parameters: add-inferior

The add-inferior command produces more output than expected when using
the native-extended-gdbserver board, because it is already connected to
a remote target:

    (gdb) add-inferior
    [New inferior 2]
    Added inferior 2 on connection 1 (extended-remote localhost:2348)

Fix that by accepting output after "Added inferior 2", as is done
elsewhere in the testsuite already (e.g. gdb.btrace/multi-inferior.exp).

Change-Id: I047a3be5249dd803839b213dd2f1646736fc8289