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

tests/basics/async_with.py

@@ -0,0 +1,17 @@
+# test simple async with execution
+
+class AContext:
+    async def __aenter__(self):
+        print('enter')
+    async def __aexit__(self, exc_type, exc, tb):
+        print('exit')
+
+async def f():
+    async with AContext():
+        print('body')
+
+o = f()
+try:
+    o.send(None)
+except StopIteration:
+    print('finished')