In Magento 2 we all follow TDD approach and we often have a requirement to use the constants and global variables.
<Magento Directory>/dev/tests/unit/phpunit.xml
The element and its children can be used to configure PHP settings, constants, and global variables.
<php>
<ini name="date.timezone" value="America/Los_Angeles"/>
<ini name="xdebug.max_nesting_level" value="200"/>
<ini name="memory_limit" value="-1"/>
<env name="BASE_URL" value="http://127.0.0.1/V1/rest/" force="true" />
</php>
If you running individual Unit Case then you need to set the configuration path. I always run them from within PHPStorm. And there is an option to define the phpunit.xml path. Preferences > Languages & Frameworks > PHP > Test Frameworks
Call the global variable in your setUp() function of the Unit Test.
protected function setUp()
{
$baseUrl = $_ENV['BASE_URL'];
}