Golang x/tools/gopls: renaming functions should rename corresponding test functions

https://github.com/user-attachments/assets/cf57b295-ab3c-4888-aca1-3dd309a521b7

Renaming a function also renames references to it. If there is a correspondingly named test function, it'd make sense to also rename it (TestHello to TestFoo when Hello renamed to Foo). Keeping the names in sync, and reducing work burden.