Use Composer-installed PHPUnit for make test

Process isolation cannot be used for test execution because ReadPreference and similar driver classes cannot be serialized (see: PHPC-850).

Makefile

 .PHONY: composer test
 
 COMPOSER_ARGS=update --no-interaction --prefer-source
-PHPUNIT_ARGS=--process-isolation
 
 composer:
 	@command -v composer >/dev/null 2>&1; \
@@ -15,12 +14,4 @@ composer:
 	fi
 
 test: composer
-	@command -v phpunit >/dev/null 2>&1; \
-	if test $$? -eq 0; then \
-		phpunit $(PHPUNIT_ARGS); \
-	elif test -r phpunit.phar; then \
-		php phpunit.phar $(PHPUNIT_ARGS); \
-	else \
-		echo >&2 "Cannot find phpunit; aborting."; \
-		false; \
-	fi
+	vendor/bin/phpunit