[Web] Add the "serve" and "run" scons targets.

You can now run the test HTTP server by calling:

scons p=web serve

If you also wish to run the browser, call instead:

scons p=web run

The default listen port is 8060, but can be overriden via the env
variable GODOT_WEB_TEST_PORT which must be a valid integer.

platform/web/SCsub

@@ -2,6 +2,20 @@
 
 Import("env")
 
+# The HTTP server "targets". Run with "scons p=web serve", or "scons p=web run"
+if "serve" in COMMAND_LINE_TARGETS or "run" in COMMAND_LINE_TARGETS:
+    from serve import serve
+    import os
+
+    port = os.environ.get("GODOT_WEB_TEST_PORT", 8060)
+    try:
+        port = int(port)
+    except Exception:
+        print("GODOT_WEB_TEST_PORT must be a valid integer")
+        sys.exit(255)
+    serve(env.Dir("#bin/.web_zip").abspath, port, "run" in COMMAND_LINE_TARGETS)
+    sys.exit(0)
+
 web_files = [
     "audio_driver_web.cpp",
     "display_server_web.cpp",