Learn English – Is the phrase “a stark contrast” redundant

Courage is reckoned the greatest of all virtues; because, unless a man has that virtue, he has no security for preserving any other.

Samuel Johnson, quote provided by Vincent McNabb in the comments.

Courage is not simply one of the virtues, but the form of every virtue at the testing point.

C.S. Lewis, sourced on Wikiquote.

To my mind, at the testing point implies a test. There's no test if you don't feel fear. I like the M-W definition you give. Note, though, that the Cambridge dictionary says that bravery is defined by not showing fear. It says nothing about not experiencing fear.

Off the top of my head, I'll say that both a brave person and a courageous person may experience fear. The brave person won't show the fear; the courageous person might. The brave are, then, a subset of the courageous. Take that with a grain of salt: I just made it up, but it sounds roughly right to me.

I take it as a metaphorical expression of Merriam-Webster's first definition: the diaresis will be used more often than usual, perhaps repeatedly and without pause, as you might subject a flabby muscle to a physical drill. I could also take it as the second, but it is a test in the sense of a trial of endurance after frequent use, not in the sense of scholarly scrutiny or academic evaluation.

I hear it often enough in conversation, e.g. This spring has been awfully wet; my galoshes have gotten a real workout, or With three kids in college, my check-writing hand gets quite a workout. We can have it in reverse as well, with something challenging giving us a workout: These fare changes are giving my brain a workout it hasn't had since algebra or These foreign names are giving my tongue a workout.