chore: add tsconfig.test.json for type-checking test files (#12828)

Adds a separate tsconfig that includes only *.test.ts files (which the main
tsconfig excludes). Available via 'pnpm tsgo:test' for incremental cleanup.

Not yet wired into 'pnpm check'  there are ~2.8k pre-existing errors in
test files that need to be fixed incrementally first.

tsconfig.test.json

@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "noEmit": true
+  },
+  "include": ["src/**/*.test.ts", "extensions/**/*.test.ts"],
+  "exclude": ["node_modules", "dist"]
+}