test: move test_peer->rx_fn function to transmit path

test: move test_peer->rx_fn function to transmit path

We currently have the peer->rx_fn called after libnvme has sent data to
the NVMe device, as a way for the test to check the sent data and modify
a response accordingly.

However, for testing More Processing Required responses, we will have
two TXes for a single RX. So, move the rx_fn callback to the TX side
(ie, when libnvme calls recvmsg). This allows the test to provide
TX-specific data for each call.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>