[tests] refactor check_last_announcement() in sendheaders.py
[tests] refactor check_last_announcement() in sendheaders.py

All calls of check_last_announcement() asserted that the return
value was True. Just assert inside the function instead. This
gives better debug information if the assert fails.

Also only check the contents of the most recent inv and header if
check_last_announcement() is called with the relevant argument.

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