[tests] fix flakiness in sendheaders.py

[tests] fix flakiness in sendheaders.py

Fixes to sources of intermittent failure in sendheaders.py

- at the start of test_null_locators(), a new block is generated and
then a getheaders is sent. check_last_accouncement() is called to assert
that the headers message is received. However, the new block triggers an
inv to be sent over both P2P connections, so there's a race. If the inv
is received at the wrong time, the test fails.

- test_null_locators() ends by sending a block to the node under test.
At the start of test_nonnull_locators(), a block is mined and
check_last_announcement() is called to assert that the inv received is
for the same block. That means there's a race: if the inv from the block
sent in test_null_locators() is received at the wrong time, the test
fails.