notmuch-test: Use named-parameters for generate_message function

notmuch-test: Use named-parameters for generate_message function

This makes the test suite bash-specific, but that's not much of
an issue for me, (if somebody else would prefer some other language
then they can rewrite the test suite and maintain it).

The advantage here is that we'll now be able to easily generate
custom messages for testing operations that depend on the message
content, (such as "notmuch reply", etc.).