Define environment variables in phpunit without using xml file

My folder structure:

tests/
    Controller/
    Library/
    Model/
    Seeds/
    bootstrap.php
    phpunit.xml

I have an env.php file that defines some variables like so:

define('TESTING',true);

but in my test files:

$this->assertTrue(defined(TESTING));

ends up being false.

My phpunit.xml file:

<?xml version="1.0" ?>
<document>
    <phpunit 
     backupGlobals="false"
     backupStaticAttributes="false"
     processIsolation="true"
     bootstrap="./bootstrap.php"
    >
        <testsuites>
            <testsuite name="Main">
                <directory>./</directory>
            </testsuite>
            <testsuite name="Library">
              <directory>./Library/</directory>
            </testsuite>
            <testsuite name="Model">
                <directory>./Model/</directory>
            </testsuite>
            <testsuite name="Controller">
                <directory>./Controller/</directory>
            </testsuite>
        </testsuites>
    </phpunit>
</document>

My bootstrap.php file requires my env.php file. So, why are the environment variables not accessible in my tests? I don't want to define all of my environment variables in my phpunit.xml file, since they are already defined in my env.php (DRY - right? ;) I have also tried using getenv(TESTING); with no success.

Thanks in advance!