Problem:
`CompletionItem.detail` is only shown in the info popup if the server
supports `completionItem/resolve`.
Solution:
If the server doesn't support `completionItem/resolve`, prepend the
complete item `info` with `CompletionItem.detail` in a fenced codeblock,
same as we do when the server supports `completionItem/resolve`.
To ensure that completion items are displayed in the same way,
regardless of whether the server supports `completionItem/resolve`, i've
extracted out the test logic from the `selecting an item triggers
completionItem/resolve + (snippet) preview` case so that we can run the
same tests against a server which supports `completionItem/resolve` and
one which doesn't. Hopefully this should prevent the two behaviours
diverging again.
d6d03716cf