accesslib: MDL-17647, MDL-17648 and MDL-17649 Bug fix, improvement and unit test.

accesslib: MDL-17647, MDL-17648 and MDL-17649 Bug fix, improvement and unit test.

MDL-17647 was referring to moodle/site:candoanything insstead of moodle/site:doanything

MDL-17648 Let get_users_by_capability take an array of capabilities, like has_any_capability

MDL-17649 get_users_by_capability must have unit tests (HEAD only).

The unit tests were briefly working (apart from the system context, which I had to set up by hand in the test contexts table). Then I made the mistake of trying to upgrade the test tables, and it all went horribly wrong (MDL-17644).