The PHPUnit testengine expects that the unit tests are located
in a folder tests or Tests. In some setups this does not fit because tests
could also be located on different places. For example they could be stored
in a folter "tests/Unit/". Therefore there should be a configuration option
for the PHPUnit tests folders instead of setting them to a fixed value.

Run arc unit --coverage in a setup where the unit test for the class
src/foo.php is stored in the folder tests/Unit/foo.php. There should be no
coverage report. Now add the configuration option to the .arcconfig

{
  "phpunit_test_dirs" : [ "tests/Unit" ]
}

and run arc unit --coverage again.