Use shared fixture repository for our tests

[vinistock]

The https://github.com/ruby-syntax-tree/ruby-syntax-fixtures repository includes an extensive list of valid Ruby snippets that is being used to test Syntax Tree and YARP.

The Ruby LSP will still need to have its own fixtures to test things such as code action refactors, where we're not only concerned about the different types of Ruby syntax, but about the behaviour in certain scenarios.

However, it would be nice to also run our requests against that repository of fixtures since it contains many examples of valid Ruby syntax we must support.

For this issue:

• Add the fixtures repository as a git submodule
• Change our expectations runner to also execute tests against the submodule fixtures
• Propose a mechanism for keeping the submodule up to date