Issue 141 fix external get users not found

[tholudwig]

This fix for #141 and #140 solves some problems in report/customsql/tests/external/external_get_users_test.php

1. renaming namespace core_external
2. hardening the output $user->id as int
3. improve comparison $defaultuserimage
4. tests run in locked processes because moodle/site:config is loaded (output in the unit tests)

Test instructions

• Run corresponding unit test --> no errors / comments

[tholudwig]

Hi,

an old version of the plugin was mistakenly checked out in our repo. After a rebase, all unit tests run without errors. The pull request is therefore not necessary.