Auto merge of #42896 - llogiq:clippy_compiletest, r=alexcrichton

Auto merge of #42896 - llogiq:clippy_compiletest, r=alexcrichton

fixed some clippy warnings in compiletest

This is mainly readability stuff. Whenever the `clone_ref` lint asked me to clone the dereferenced object, I removed the `.clone()` instead, relying on the fact that it has worked so far and the immutable borrow ensures that the value won't change.