PHPUnit crashes when test suite contains both *.phpt files and unconventionally named tests

[narkq]

Q	A
PHPUnit version	7.0-g3e2ff3bb4 6.5.5 5.7.26
PHP version	7.1.11
Installation Method	Composer

Code for reproducing:
https://github.com/narkq/phpunit-hello-world

Expected result:
All tests pass.

Actual result:

PHP Fatal error:  Uncaught PHPUnit\Framework\Exception: Class "PHPUnit\Runner\PhptTestCase" does not extend PHPUnit\Framework\TestCase.

  thrown in /home/sladkov/src/phpunit-hello-world/vendor/phpunit/phpunit/src/Framework/TestSuite.php on line 158

composer info | sort output:

doctrine/instantiator              dev-master 8520afa A small, lightweight utility to instantiate objects in PHP without invoking their constructors
myclabs/deep-copy                  1.x-dev 9f80720    Create deep copies (clones) of your objects
phar-io/manifest                   dev-master 014fead Component for reading phar.io manifest information from a PHP Archive (PHAR)
phar-io/version                    1.0.1              Library for handling version information and constraints
phpdocumentor/reflection-common    dev-master e8e68dc Common reflection classes used by phpdocumentor to reflect the code structure
phpdocumentor/reflection-docblock  dev-master 86868b9 With this component, a library can provide support for annotations via DocBlocks or otherwise retrieve information that is embedded...
phpdocumentor/type-resolver        0.5.0              
phpspec/prophecy                   dev-master 9a84d1c Highly opinionated mocking framework for PHP 5.3+
phpunit/php-code-coverage          dev-master 66a25f3 Library that provides collection, processing, and rendering functionality for PHP code coverage information.
phpunit/php-file-iterator          1.4.x-dev 730b01b  FilterIterator implementation that filters files based on a list of suffixes.
phpunit/php-text-template          1.2.1              Simple template engine.
phpunit/php-timer                  dev-master e73a731 Utility class for timing
phpunit/php-token-stream           dev-master a60fc2a Wrapper around PHP's tokenizer extension.
phpunit/phpunit                    dev-master 3e2ff3b The PHP Unit Testing framework.
phpunit/phpunit-mock-objects       dev-master 74826a1 Mock Object library for PHPUnit
sebastian/code-unit-reverse-lookup dev-master 3488be0 Looks up which function or method a line of code belongs to
sebastian/comparator               dev-master c2019a4 Provides the functionality to compare PHP values for equality
sebastian/diff                     dev-master 0fd4d14 Diff implementation
sebastian/environment              dev-master ceaeb5d Provides functionality to handle HHVM/PHP environments
sebastian/exporter                 dev-master 573f8b7 Provides the functionality to export PHP variables for visualization
sebastian/global-state             dev-master a27e666 Snapshotting of global state
sebastian/object-enumerator        dev-master a496797 Traverses array structures and object graphs to enumerate all referenced objects
sebastian/object-reflector         dev-master ff75508 Allows reflection of object attributes, including inherited and non-public ones
sebastian/recursion-context        dev-master 0f7f5eb Provides functionality to recursively process PHP variables
sebastian/resource-operations      dev-master fadc83f Provides a list of PHP built-in functions that operate on resources
sebastian/version                  dev-master 99732be Library that helps with managing the version number of Git-hosted PHP projects
theseer/tokenizer                  1.1.0              A small library for converting tokenized PHP source code into XML and potentially other formats
webmozart/assert                   dev-master 4a8bf11 Assertions to validate method input/output with nice error messages.