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

tests/basics/async_for2.py

@@ -0,0 +1,48 @@
+# test waiting within "async for" aiter/anext functions
+
+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 f(x):
+    print('f start:', x)
+    yield x + 1
+    yield x + 2
+    return x + 3
+
+class ARange:
+    def __init__(self, high):
+        print('init')
+        self.cur = 0
+        self.high = high
+
+    async def __aiter__(self):
+        print('aiter')
+        print('f returned:', await f(10))
+        return self
+
+    async def __anext__(self):
+        print('anext')
+        print('f returned:', await f(20))
+        if self.cur < self.high:
+            val = self.cur
+            self.cur += 1
+            return val
+        else:
+            raise StopAsyncIteration
+
+async def coro():
+    async for x in ARange(4):
+        print('x', x)
+
+o = coro()
+try:
+    while True:
+        print('coro yielded:', o.send(None))
+except StopIteration:
+    print('finished')