Why PhpStorm doesn't know PHPUnit_Framework_TestCase

I'm trying to configure PhpStorm 2017.2 to use PhpUnit 5 for my PHP 5.6 project.

1. I've downloaded the phpunit-5.7.21.phar file from the official source and placed it in my PHP 5.6 installation dir.

2. In PhpStorm Settings >> Languages & Frameworks >> PHP >> Test Frameworks, I've linked to the .phar executable and set the default config file to a phpunit.xml in the project root directory

3. Here are the contents of phpunit.xml:

.

<?xml version="1.0" encoding="UTF-8"?>

<phpunit>
    <testsuites>
        <testsuite name="Test suite">
            <directory>tests</directory>
        </testsuite>
    </testsuites>
</phpunit>

I'm trying to structure tests in a tests/unit directory within which my source file project structure would be mirrored as described in the manual. For instance:

// project files:
ClassOne.php
vendor/
  ClassTwo.php
  Utility.php

// test files
tests/unit/
  ClassOneTest.php
  vendor/
    ClassTwoTest.php
    UtilityTest.php

I have two problems though:

First, I don't know how to configure PhpStorm to create tests within tests/unit/ mirroring the sructure with respect to the project root. When I create a test, by default the file is put in the same directory as the project file.

Secondly, I don't know how to get PhpStorm to index the PHPUnit source code. Even though I've linked to phpunit-5.7.21.phar file as shown above, the IDE complains when I create a test:

namespace vendor;
class UtilityTest extends \PHPUnit_Framework_TestCase{}

Undefined class PHPUnit_Framework_TestCase

Update 1

I solved the 2nd problem by adding the directory where I had saved the .phar to the PhpStorm include path, set in Settings >> Languages & Frameworks >> PHP >> Include Path tab. Alternatively, I could just put the .phar file within the project directory and it will be indexed.

I still need help with my first problem.

Update 2

Thanks to Ástþór's answer I figured out how to get PhpStorm to mirror the project structure within a dedicated tests directory. Go to PhpStorm Settings >> Directories and select the base testing directory. The click Test near the top to mark it as a Test Sources Root

The next time you create a test, it will automatically be placed in that directory.