Allow phpunit tests to run without .env file

Summary:
D253 has reverted D141, and so by default APP_KEY is invalid, forcing
a setup operation to ensure a proper unique key is generated.

D141 goal were to allow phpunit to run without an .env file.

We can achieve this goal adding a dummy APP_KEY in phpunit environment.

Test Plan: Run phpunit tests

Reviewers: dereckson

Subscribers: alken-orin, Kaliiixx

Maniphest Tasks: T691

Differential Revision: http://devcentral.nasqueron.org/D255