btrace, testsuite: fix extended-remote non-stop test

With --target_board=native-extended-gdbserver non-stop tests are failing with

    UNTESTED: gdb.btrace/non-stop.exp: failed to run to main

Fix that by adding '-ex "set non-stop on"' to GDBFLAGS before restarting.

testsuite/
	* gdb.btrace/non-stop.exp: Add '-ex "set non-stop on"' to GDBFLAGS.