webassembly/proxy_js: Promote Python thenable to a Promise.

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

tests/ports/webassembly/fun_py_callback_js.mjs

@@ -0,0 +1,31 @@
+// Test using Python functions as JS callbacks.
+
+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")
+
+async def f1():
+    print("f1 run")
+
+async def f2():
+    print("f2 start")
+    await js.asyncTimeout(0)
+    print("f2 end")
+
+async def f3():
+    print("f3 start")
+    await f2()
+    print("f3 end")
+
+js.setTimeout(f0, 0)
+js.setTimeout(f1, 50)
+js.setTimeout(f2, 100)
+js.setTimeout(f3, 150)
+`);

tests/ports/webassembly/fun_py_callback_js.mjs.exp

@@ -0,0 +1,8 @@
+f0 run
+f1 run
+f2 start
+f2 end
+f3 start
+f2 start
+f2 end
+f3 end