This PR fixes an issue on new NeoVim versions that would cause the
language server to display an error when using certain code actions.
(For some reason, NeoVim recently decided to diverge from VS Code in
terms of when it emits code action resolution requests, which means that
not setting the `data?` field won't preclude NeoVim from emitting a
request anymore, which in turn means that the server can't resolve the
code action.)
This PR significantly improves the test coverage of the language server,
providing at least a single basic test for every request that is used by
the client. It also implements infrastructure for testing all of these
requests, e.g. the ability to run interactive tests in a project context
and refactors the interactive test runner to be more maintainable.
Finally, it also fixes a small bug with the recently implemented unknown
identifier code actions for auto-implicits (#10442) that was discovered
in testing, where the "import all unambiguous unknown identifiers" code
action didn't work correctly on auto-implicit identifiers.
