yocto/safety_island: Fix race condition in Zephyr client tests

In the Zephyr client HIPC tests the client is being launched immediately after
the server. This means that the server may not have fully started before the
client sends its first packet (depending on the relative performance of the
Primary Compute and the Safety Island in the model). This has led to
intermittent UDP packet loss and TCP connection issues under test.

Fix this issue by waiting for the server to printed "Server listening..."
before attempting to start the client, in both the Zephyr UDP and TCP client
tests.

Issue-Id: SCM-5864
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I3ace5ecb066e4fb6874cfc82806106c557d11cb8