Example PHPUnit test with UnitTestCase testing a method of a utility class https://gist.github.com/kevinquillen/d7efe7e2bce417289f8fabc2e2f1fe62

EmailMatcherTest.php

<?php

namespace Drupal\Tests\iana_netforum_auth\Unit;

use Drupal\Tests\UnitTestCase;
use Drupal\iana_netforum_auth\Utility\EmailMatcher;

/**
 * Test email matching component class.
 *
 * @group iana_netforum_auth
 *
 * @coversDefaultClass \Drupal\iana_netforum_auth\Utility\EmailMatcher
 */
class EmailMatcherTest extends UnitTestCase {

  /**
   * Test that the EmailMatcher method returns what we expect.
   *
   * @dataProvider providerTestMatch
   * @covers ::emailDomainMatch
   */
  public function testMatch($email_address, $domain, $expected) {
    $this->assertEquals($expected, EmailMatcher::emailDomainMatch($email_address, $domain));
  }

  /**
   * Data provider for testMatch().
   *
   * @see testMatch()
   *
   * @return array
   *   An array containing:
   *     - The email address provided.
   *     - The domain we want to check.
   *     - The bool that should be returned.
   */
  public function providerTestMatch() {
    return [
      ['tester@velir.com', 'velir.com', TRUE],
      ['tester@velr.com', 'velir.com', FALSE],
      ['tester@gmail.com', 'velir.com', FALSE],
      ['velir.com', 'velir.com', FALSE],
      ['velir@velir.org', 'velir.com', FALSE],
      ['tester@velir.com@gmail.com', 'velir.com', FALSE],
      ['tester@velir.com@velir.com', 'velir.com', FALSE]
    ];
  }
}