tests: coap_client: optimize/reduce sleep time

The goal of this commit is to reduce the overall test time by optimizing
the time spent sleeping.

However while doing so, a few glitches became apparent and those have
been fixed as well.

1. The way the ZSOCK_POLLIN event is managed has been modified

In most tests, the event would stick for the whole test and this would
sometimes results in the client receiving more data than intended which
in turn would results in various unexpected warnings and errors.

The management of the ZSOCK_POLLIN event has now been mostly moved to
the send/receive overrides which better represent how things would
happen outside of the test scenario.

For example, when sending data, if this send would normally result in
receiving some response, the send function sets the ZSOCK_POLLIN event.
Then when receiving, we clear the event as the data has been received.

There are a few exceptions to this for cases where we need to simulate
some specific abnormal behavior.

2. The `messages_needing_response` queue now includes a valid bit

The test manages a queue of messages id to be able to respond to the
correct id in the receive hooks.

It was built in a way that the id 0 would be treated as invalid
although it is a valid id.
In practice, it would not be an issue in most cases however, it would
result in an unexpected behavior if the `test_multiple_requests` test
would be run first.

To fix this issue in the simplest way, the queue has been changed to
uint32_t which gives us 16 extra bits for the management of the queue,
1 of which is used to tell if the entry is valid or not.

Signed-off-by: Francois Gervais <francoisgervais@gmail.com>