x/tools/gopls: offer test function snippets as completion candidates in test files

[findleyr]

Suggestion by way of @pjweinb: when typing a new function declaration in a test file (*_test.go), we could offer completion items for untested exported functions in the non-test package.

For example, if package p defines function Foo, we could offer the completion suggestion func TestFoo when typing in p_test.go.

Extra credit: populate the completion snippet with a test body that sets up a table-driven test for the function in question.

[robbiesumner]

I think this is a great idea. 👍🏼 I could maybe work on this.

[gopherbot]

Change https://go.dev/cl/554836 mentions this issue: gopls: offer test function completion for untested methods

[findleyr]

@robbiesumner this might be tricky to implement, but you are welcome to give it a go. You'll need to walk the package scope looking at top level declarations, and pairing off exported methods with existing tests.

Let us know if you have any questions, or have anything to review.