When doing Twig tests extending Twig\Test\IntegrationTestCase there is a skipped test if no legacy tests are defined:

There was 1 skipped test:

1) MyTests\IntegrationTest::testLegacyIntegration with data set #0 ('not', '-', '', array(), '', array())
no tests to run

This is coming from testLegacyIntegration in IntegrationTestCase, and specifically the beginning of doIntegrationTests:

if (!$outputs) {
    $this->markTestSkipped('no tests to run');
}