tests: Add 6 tests for async await/for/with.

tests/basics/async_await2.py

@@ -0,0 +1,27 @@
+# test await expression
+
+import sys
+if sys.implementation.name == 'micropython':
+    # uPy allows normal generators to be awaitables
+    coroutine = lambda f: f
+else:
+    import types
+    coroutine = types.coroutine
+
+@coroutine
+def wait(value):
+    print('wait value:', value)
+    msg = yield 'message from wait(%u)' % value
+    print('wait got back:', msg)
+    return 10
+
+async def f():
+    x = await wait(1)**2
+    print('x =', x)
+
+coro = f()
+print('return from send:', coro.send(None))
+try:
+    coro.send('message from main')
+except StopIteration:
+    print('got StopIteration')