Fix test fixtures for opa and add some comments.

test/fixtures/hello_syntax1.opa

@@ -1,5 +1,9 @@
-server = Server.one_page_bundle("Hello World",
-       [],
-       [],
-       _ -> <h1>Hello, world</h1>)
+/**
+ * To compile & run on port 8080:
+ * opa hello_syntax1.opa --
+ */
+server = Server.one_page_server(
+   "Hello, world",
+   -> (<h1>Hello, world</h1>)
+)
 

test/fixtures/hello_syntax2.opa

@@ -1,3 +1,7 @@
+/**
+ * To compile & run on port 8080:
+ * opa --parser js-like hello_syntax2.opa --
+ */
 Server.start(
    Server.http,
    {