Unit testing IValueConverter
This is a good unit test. It may seem like a trivial case to test a boolean response, but wrapping this in a test ensures that you always get the correct response. What if you decide to refactor the internals of BooleanToVisibilityConverter down the road? With this test, you can always be confident that it's behaving correctly.

To me, the two assertions look like they belong together. As a matter of style, you could split the test up so that you have two tests with a straight AAA (Arrange, Act, Assert) pattern - one for true, one for false. In my opinion, that would be a little excessive. I like a test that looks like this because it is clear and quick.

If you were writing a test of a converter that had many possible outputs and multiple input conditions (for example), then you definitely should consider splitting it up and testing each condition separately. For now, you're on the right track.