How to run tests in my contrib module?
You have to include phpunit/phpunit package in composer, alredy done if you took default drupal installation :

composer require phpunit/phpunit --dev


A file should be filled which describe yours testsuites, generally phpunit.xml

It looks like this ./phpunit.xml :

&lt;phpunit bootstrap="web/core/tests/bootstrap.php" colors="true"
         beStrictAboutTestsThatDoNotTestAnything="true"
         beStrictAboutOutputDuringTests="true"
         beStrictAboutChangesToGlobalState="true"
         checkForUnintentionallyCoveredCode="false"
         printerClass="\Drupal\Tests\Listeners\HtmlOutputPrinter"&gt;

&lt;php&gt;
  &lt;ini name="error_reporting" value="32767"/&gt;
  &lt;ini name="memory_limit" value="-1"/&gt;
  &lt;env name="SIMPLETEST_BASE_URL" value="http://drupal-8.localhost"/&gt;
  &lt;env name="SIMPLETEST_DB" value="mysql://drupal-8:drupal-8@localhost/drupal-8"/&gt;
  &lt;env name="BROWSERTEST_OUTPUT_DIRECTORY" value="/var/www/sites/default/simpletest"/&gt;
&lt;/php&gt;
  &lt;testsuites&gt;
    &lt;testsuite name="testname1"&gt;
      &lt;directory&gt;web/modules/custom/your_module/tests&lt;/directory&gt;
    &lt;/testsuite&gt;
    &lt;testsuite name="testname2"&gt;
      &lt;directory&gt;web/modules/custom/your_module2/tests&lt;/directory&gt;
    &lt;/testsuite&gt;
  &lt;/testsuites&gt;
&lt;/phpunit&gt;


In this example, drupal is located in ./web/ directory with ./vendor/, both in root path (./) .

You can add test for contrib module, but they are already tested (if they contain tests) when authors are pushing their sources to drupal.org ( https://www.drupal.org/pift-ci-job/1339317 ) :

&lt;testsuite name="toolbar"&gt;
    &lt;directory&gt;./web/modules/contrib/admin_toolbar/tests/src/FunctionalJavascript&lt;/directory&gt;
&lt;/testsuite&gt;


Then you can run tests with :

./vendor/phpunit/phpunit/phpunit


To use a specific description file :

./vendor/phpunit/phpunit/phpunit -c phpunit-other.xml


To run a specific testsuite :

./vendor/phpunit/phpunit/phpunit --testsuite=testname1