Add test for drop order in async functions.
Add test for drop order in async functions.

This tests that async functions drop parameters in the same order as
regular functions.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust