BOOST_TEST_MESSAGE no longer works. see: #18472
Since it doesn't seem like BOOST_TEST_MESSAGE was missed, instead of replacing with cout maybe it's preferable to just remove the test messages. For now I just put cout in the place of BOOST_TEST_MESSAGE.
Also as a consequence of #18472, log_level option can no longer be passed via the cli so this PR removes it from the docs.
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Can you visualize the changes with an example command and the previous output on master and the new output on this pull?
@MarcoFalke I don't understand why, but now BOOST_TEST_MESSAGE is working on my system. I'll close this PR for now.