Gist for https://stackoverflow.com/questions/57145143/how-to-mock-use-or-override-environmentiscli-in-typo3-v9-unit-tests

phpunit.xml

<phpunit bootstrap="vendor/autoload.php">
	<testsuites>
		<testsuite name="Tests">
			<directory>Tests</directory>
		</testsuite>
	</testsuites>

	<filter>
		<whitelist processUncoveredFilesFromWhitelist="true">
			<directory>Classes</directory>
		</whitelist>
	</filter>
</phpunit>

RequestHandlerHook.php

<?php

	namespace My\NamespaceRedirect\Tests\Unit\Hooks;

	use My\NamespaceRedirect\Hooks\RequestHandlerHook;
	use My\NamespaceRedirect\Service\DomainDataService;
	use My\NamespaceRedirect\Service\HeaderService;
	use My\NamespaceRedirect\Service\RedirectDataService;
	use TYPO3\CMS\Core\Core\Environment;
	use TYPO3\CMS\Core\Core\SystemEnvironmentBuilder;
	use TYPO3\CMS\Core\Utility\GeneralUtility;
	use TYPO3\CMS\Core\Utility\HttpUtility;
	use TYPO3\TestingFramework\Core\Unit\UnitTestCase;

	class RequestHandlerHookTest extends UnitTestCase {

		/**
		 * @var \PHPUnit_Framework_MockObject_MockObject|DomainDataService
		 */
		protected $domainDataServiceMock;

		/**
		 * @var \PHPUnit_Framework_MockObject_MockObject|RedirectDataService
		 */
		protected $redirectDataServiceMock;

		/**
		 * @var \PHPUnit_Framework_MockObject_MockObject|HeaderService
		 */
		protected $headerServiceMock;

		/**
		 * @var RequestHandlerHook
		 */
		protected $requestHandlerHookSUT;

		protected $backupEnvironment = TRUE;

		protected function setUp() {
			define("TYPO3_PATH_WEB", "");
			define("PATH_thisScript", "./Tests/Unit/Hooks/RequestHandlerHookTest.php");
			SystemEnvironmentBuilder::run();

			$this->resetSingletonInstances = TRUE;

			parent::setUp();

			$this->domainDataServiceMock = $this->createMock(DomainDataService::class);
			GeneralUtility::setSingletonInstance(DomainDataService::class, $this->domainDataServiceMock);

			$this->redirectDataServiceMock = $this->createMock(RedirectDataService::class);
			GeneralUtility::setSingletonInstance(RedirectDataService::class, $this->redirectDataServiceMock);

			$this->headerServiceMock = $this->createMock(HeaderService::class);
			GeneralUtility::addInstance(HeaderService::class, $this->headerServiceMock);

			$this->requestHandlerHookSUT = new RequestHandlerHook();
		}

		public function testHandleRequest() {
			Environment::initialize(
				Environment::getContext(),
				TRUE,
				FALSE,
				Environment::getProjectPath(),
				Environment::getPublicPath(),
				Environment::getVarPath(),
				Environment::getConfigPath(),
				Environment::getBackendPath().'/index.php',
				Environment::isWindows() ? 'WINDOWS' : 'UNIX'
			);

			$path = "seite.html";
			$domain = "www.mydomain.de";
			$expectedStartPageId = 3;
			$redirectUid = 45;
			$redirectDestination = "https://www.foreignDomain.de";

			$GLOBALS["TYPO3_CONF_VARS"]["SYS"] = [
				"reverseProxySSL" => "",
				"reverseProxyIP" => "",
				"requestURIvar" => FALSE
			];
			$_SERVER["REMOTE_ADDR"] = "127.0.0.1";
			$_SERVER["SSL_SESSION_ID"] = "";
			$_SERVER["ORIG_SCRIPT_NAME"] = "";
			$_SERVER["SCRIPT_NAME"] = "/index.php";
			$_SERVER["HTTPS"] = "on";
			$_SERVER["HTTP_HOST"] = $domain;
			$_SERVER["REQUEST_URI"] = "/$path";

			$this->domainDataServiceMock
				->expects($this->once())
				->method("getStartPageId")
				->with($domain)
				->willReturn($expectedStartPageId);

			$this->redirectDataServiceMock
				->expects($this->once())
				->method("findBySource")
				->with($path, $expectedStartPageId)
				->willReturn([
					"uid" => $redirectUid,
					"destination" => $redirectDestination
				]);

			$this->headerServiceMock
				->expects($this->once())
				->method("redirect")
				->with($redirectDestination, HttpUtility::HTTP_STATUS_301);

			$this->requestHandlerHookSUT->handleRequest();
		}
	}