Decide upon unit test namespace rules

While I have been unable to find it, we have asked already a number of times for unit tests not to use namespaces. This is about try to ensure that's observed:

• Document it more clearly, right now there are 3 main pages (that, indeed may need a refactor, apart from this issue):
  • https://docs.moodle.org/dev/PHPUnit
  • https://docs.moodle.org/dev/Writing_PHPUnit_tests
  • https://docs.moodle.org/dev/PHPUnit_integration
• Check for that in local_ci phpunit execution.
• Fix current violations.

  $ ag '^namespace' | grep _test.php

  lib/form/tests/external_test.php:26:namespace core_form;

  lib/form/tests/filetypes_util_test.php:26:namespace core_form;

  webservice/soap/tests/wsdl_test.php:25:namespace webservice_soap;

Ciao