Basic Example 1:

declare(strict_types=1);

final class FunctionsTest extends WP_UnitTestCase {
  public function testPushAndPop() {
      $stack = [];
      $this->assertSame(0, count($stack));

      array_push($stack, 'foo');
      $this->assertSame('foo', $stack[count($stack)-1]);
      $this->assertSame(1, count($stack));

      $this->assertSame('foo', array_pop($stack));
      $this->assertSame(0, count($stack));
  }

}

Basic Example 2:
with setUp() & tearDown().

use PHPUNIT_Framework_TestCase as TestCase;
// sometimes it can be
// use PHPUnit\Framework\TestCase as TestCase;

class MathTest extends TestCase {
    public $fixtures;
    protected function setUp() {
        $this->fixtures = [];
    }

    protected function tearDown() {
        $this->fixtures = NULL;
    }

    public function testEmpty() {
        $this->assertTrue($this->fixtures == []);
    }
}
 

For more info:
https://riptutorial.com/phpunit