x/tools/gopls: add source test for inlay hints

[suzmue]

Track progress of source tests for inlay hints.

[sidntrivedi012]

Hey @suzmue 👋 , would like to help out with this issue. Would be great if you can please provide some more context regarding what needs to be done here. Thanks! 🙌

[suzmue]

With the addition of features that are exposed through inlay hints, we may want to add source tests for these features (code pointer here).

These tests would need to call the source package's implementation here.

The lsp tests currently take the approach of getting the inlay hints for the whole file and then applying them to the document as if they were text edits. It will probably require some additional set up to be able to track what inlay hints we are expecting from these source tests.