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

tests/basics/async_for.py

@@ -0,0 +1,29 @@
+# test basic async for execution
+# example taken from PEP0492
+
+class AsyncIteratorWrapper:
+    def __init__(self, obj):
+        print('init')
+        self._it = iter(obj)
+
+    async def __aiter__(self):
+        print('aiter')
+        return self
+
+    async def __anext__(self):
+        print('anext')
+        try:
+            value = next(self._it)
+        except StopIteration:
+            raise StopAsyncIteration
+        return value
+
+async def coro():
+    async for letter in AsyncIteratorWrapper('abc'):
+        print(letter)
+
+o = coro()
+try:
+    o.send(None)
+except StopIteration:
+    print('finished')