Laravel 5 testing environment not getting set in Codeception unit tests

I'm using Laravel 5 and Codeception and I would like to use an in-memory SQLite database for my unit tests, however, I can't seem to get my environment set to "testing" in Codeception. I'm using the Laravel5 module and have the following defined in my unit.suite.yml file:

class_name: UnitTester
modules:
    enabled: [Asserts, UnitHelper, Laravel5]
    config:
        Laravel5:
            environment_file: .env.testing

I have a .env file which defines all my local settings, then a .env.testing file that defines all the testing-specific settings. However, it never seems to actually set the environment correctly.

To test the environment I just did:

$this->assertEquals('testing', \App::environment());

and I always get:

Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'testing'
+'local'

Anyone have any idea what I'm doing wrong?