Renamed DrupalTestBase and UnitTestCase::randomName() to randomMachineName()

DrupalTestBase and UnitTestCase include a helper called randomName() for generating a random machine name.

randomName() does not entirely clarify the function's purpose: to generate a random machine name, for use generating already-validated machine names.

randomMachineName() is more clear on the purpose on the method, avoiding confusion with randomString(). This common confusion has a history of providing random errors because of wrong usage because the difference is not well spot for novices.

Before

$action_label = $this->randomName();

After

$action_label = $this->randomMachineName();