test: move test-debugger-debug-brk to sequential

Move test from `test/debugger` to `test/sequential` so that it is
exercised by CI and `make test`.

PR-URL: #6731
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott