cam-service/test: Increment delay before trying to connect

A spurious error is happening where the connect function inside
test_socket_single_client is failing with error code 111
(connection refused), it means that the server is not ready yet
to accept connections, so increase the delay before connecting.

While there, implement also a retry mechanism to improve the
robustness of the test.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>