testing: implement related code (#222252)

* wip

* testing: implement related code

This implements a simple provider-based related code feature, and
adopts it in the selfhost test extension.

It's surfaced to users via automatic delegation in the "run tests at
cursor" command, which I feel like it very natural. I also add two
commands which reuse the `SymbolNavigationAction` to implement
go to/peek views using the references infrastructure. The upstream
change involved in this is the addition of the "go to" preference
for tests.

cc @jrieken @mjbvz

* bump distro for API enablement

.vscode/extensions/vscode-selfhost-test-provider/package.json

@@ -4,6 +4,7 @@
   "description": "Test provider for the VS Code project",
   "enabledApiProposals": [
     "testObserver",
+    "testRelatedCode",
     "attributableCoverage"
   ],
   "engines": {
@@ -83,6 +84,7 @@
   "dependencies": {
     "@jridgewell/trace-mapping": "^0.3.25",
     "ansi-styles": "^5.2.0",
+    "cockatiel": "^3.1.3",
     "istanbul-to-vscode": "^2.0.1"
   }
 }