Add a public initializer for `ServerSessionTestStub`, to make it actually usable in tests.

Sources/SwiftGRPC/Runtime/ServerSession.swift

@@ -48,4 +48,6 @@ open class ServerSessionTestStub: ServerSession {
   open var statusMessage = "OK"
   open var initialMetadata = Metadata()
   open var trailingMetadata = Metadata()
+
+  public init() {}
 }