client_borrow_error test is failing

[ehuss]

The client_borrow_error test is failing on latest nightly. This is because the 2015 edition now includes NLL as of rust-lang/rust#59114.

If this test is just verifying that it can handle an error diagnostic, perhaps it could just be replaced with the compile_error! macro?

[alexheretic]

Realistic scenario tests are nice though, I think we can rise to the task of producing a borrow error post-nll.