Support member references in AssertJ assertThrows

[Meijuh]

This PR adds support for rewriting JUnit assertions to AssertJ assertions for throwing member references.
Lambdas in the form of () -> { throw new Exception(); } were already supported. Now var::throwingMemberReference is also supported.

I'm not really sure what rewrite's default behavior is for unsupported semantics; the added code simply does not rewrite unsupported semantics. Alternatively, the PR can be adjusted to throw an IllegalArgumentException in the last if-else branch.

[timtebeek]

Clear improvement to cover a case we had not identified before; thanks a lot!

[timtebeek]

I'm not really sure what rewrite's default behavior is for unsupported semantics; the added code simply does not rewrite unsupported semantics. Alternatively, the PR can be adjusted to throw an IllegalArgumentException in the last if-else branch.

We strive to "do no harm", as you might have discovered in the docs already; that often means not making a change is preferred over making a partial change. Sometimes we add in a comment when we can not convert a particular case where conversion is necessary; in this case I think it's fine to leave in the original statement, as is implied by the fall through return mi;.

Throwing an IllegalArgumentException is typically undesirable; we want to allow users to confidently run recipes against large code bases; uncovered cases are ideally then best not converted rather than breaking a recipe run that might be chained into a larger composite recipe.

Thanks for bringing this case to our attention, and providing an immediate fix as well!