webassembly/proxy_c: Ensure return value of async fun is passed to JS.

Signed-off-by: Damien George <damien@micropython.org>

tests/ports/webassembly/await_js_async_py.mjs

@@ -0,0 +1,35 @@
+// Test JavaScript await'ing on Python async functions.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+globalThis.asyncTimeout = (ms) =>
+    new Promise((resolve) => setTimeout(resolve, ms));
+
+mp.runPython(`
+import js
+
+def f0():
+    print("f0 run")
+    return 1
+
+async def f1():
+    print("f1 run")
+    return 2
+
+async def f2():
+    print("f2 start")
+    await js.asyncTimeout(0)
+    print("f2 end")
+    return 3
+
+async def f3():
+    print("f3 start")
+    ret = await f2()
+    print("f3 end")
+    return ret + 1
+`);
+
+console.log("f0 return:", await mp.globals.get("f0")());
+console.log("f1 return:", await mp.globals.get("f1")());
+console.log("f2 return:", await mp.globals.get("f2")());
+console.log("f3 return:", await mp.globals.get("f3")());

tests/ports/webassembly/await_js_async_py.mjs.exp

@@ -0,0 +1,12 @@
+f0 run
+f0 return: 1
+f1 run
+f1 return: 2
+f2 start
+f2 end
+f2 return: 3
+f3 start
+f2 start
+f2 end
+f3 end
+f3 return: 4