Rollup merge of #55801 - pnkfelix:update-box-insensitivity-test-for-nll, r=davidtwco

Rollup merge of #55801 - pnkfelix:update-box-insensitivity-test-for-nll, r=davidtwco

NLL: Update box insensitivity test

This is just keeping one of our tests honest with respect to NLL, in two ways:

 1. Adds uses of borrows that would otherwise be too short to observe the error that we would have expected to see...
 2. ... I say "would have expected" because all of the errors in this file are part of the reversion of rust-lang/rfcs#130 that is attached to NLL (you can see more discussion of this here https://github.com/rust-lang/rust/issues/43234#issuecomment-411017768 )