Unable to run tests via the simpletest UI

[danepowell]

My system information:

• BLT version: 8.8.5

I'm running a fresh install of a BLT-based project. When I enable the core simpletest module and then try to run this test:

Drupal\Tests\search_api_solr\Kernel\SearchApiSolrTest

I get 9 errors like this:

1. Drupal\Tests\search_api_solr\Kernel\SearchApiSolrTest::testBackend
   PHPUnit_Framework_Exception: PHP Fatal error: Class 'org\bovigo\vfs\vfsStream' not found in docroot/core/tests/Drupal/KernelTests/KernelTestBase.php on line 287
   Fatal error: Class 'org\bovigo\vfs\vfsStream' not found in docroot/core/tests/Drupal/KernelTests/KernelTestBase.php on line 287

I think this is a similar root cause to #1047, #1048, and #1105, although those had to do with running PHPUnit tests from the command line, and I think it should be possible to run them from the simpletest UI as well.

[danepowell]

The reason for this seems to be a missing composer package (vfsstream) for reasons explained here: https://www.drupal.org/node/2889302

While I think Drupal should fix this in the long term, I think BLT should also work around it in the short term by just adding this dependency itself, the same way DrupalVM did. PR incoming.