Accommodate currently-supported version of PHPUnit

[clarkphp]

This PR makes the unit tests compatible with PHPUnit 9. It considers the following information:

• PHP current stable is version 8.0, old stable is version 7.4, PHP 7.3 is not under active support, and receives only security patches through December 6 2021, at which point it reaches end-of-life. Earlier versions of PHP are already at end-of-life. See https://www.php.net/supported-versions.php
• Currently supported PHPUnit version 9 requires PHP 7.3 or newer, and receives bug fixes until February 4, 2022. See https://phpunit.de/announcements/phpunit-9.html
• PHPUnit version 8, though it supports PHP 7.2 or newer, receives bug fixes only until February 5, 2021. See https://phpunit.de/announcements/phpunit-8.html
• The unit tests appear to have been written using PHPUnit 5.

The minor changes are as follows:

• Update phpunit configuration to prevent warnings
• Version 9.3.0 introduced changes to the configuration file that should be accommodated. https://github.com/sebastianbergmann/phpunit/blob/a0d6b21c6c8f6564212a1a14292d230ee35eba6d/ChangeLog-9.3.md#930---2020-08-07
• Replace PHPUnit_Framework_TestCase with \PHPUnit\Framework\TestCase
• Replace $this->setExpectedException() with $this->expectException(\Exception::class);