Add expecttest to requirements.txt (pytorch#63320)

Summary:
This PR closes the developer environment gap left by pytorch#60658 by adding [expecttest](https://github.com/ezyang/expecttest) to `requirements.txt`. Thus it provides a solution to one of the short-term problems that pytorch#60697 tries to solve, but does not provide a long-term solution to pytorch#61375.

Pull Request resolved: pytorch#63320

Reviewed By: malfet

Differential Revision: D30340654

Pulled By: samestep

fbshipit-source-id: 26c8f8c9889cce4a94fafb1bf2f0d6df4c70503f

requirements.txt

@@ -1,5 +1,6 @@
 # Python dependencies required for development
 astunparse
+expecttest
 future
 numpy
 psutil