feat(companion): serve the wire protocol from a parked runner

companion/Sources/RunnerTestCase.swift

@@ -0,0 +1,11 @@
+import XCTest
+
+// The single long-lived test method: it starts the server and parks the runner
+// in a run loop so the simulator process stays alive serving the protocol.
+final class RunnerTestCase: XCTestCase {
+    func testServeForever() throws {
+        let server = try Server()
+        server.start()
+        RunLoop.current.run()
+    }
+}