tests/boards/intel_adsp/smoke: Wait for host script to be ready

These tests work in tandem with the cavstool.py script: the script
receives IPC messages and properly acts on them. However, it may take
some time to the script execution arrive at the point in which it's
ready to receive incoming messages. This patch makes the test code wait
some arbitrary time to allow the script to reach the desired state.

Note that another IPC message could be used to this end, but since some
tests are actually testing the IPC mechanism, it seems unwise to assume
IPC works for set up steps.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>