[tests] Fix p2p_sendheaders race
[tests] Fix p2p_sendheaders race

p2p_sendheaders has a race in part 1.3.

part 1.2 sends a block to the node over the 'test_node' connection, but
doesn't wait for an inv to be received on the 'inv_node' connection. If
we get to part 1.3 before that inv has been received, then the
subsequent call to check_last_inv_announcement could fail.

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