Assignment test rewrite

The assignment tests are full of needless bloat.

It appears that all assignment tests extend base, which has a ridonculous setUp which is not used in a great number of the tests. It's also creating a course, 7 users, and 6 groups, and requiring resetAfterTest() on each test. None of the above are cheap.

The time taken for my machine to run the full suite is ~ 15 minutes; and 2 minutes of that is just on mod_assign.

With the amount of setUp here, I suspect that we can probably halve time used for mod_assign.
I also suspect that many of them aren't actual unit tests, but behavioural.

2037 sm:MDL-54872-master> ./vendor/bin/phpunit --testsuite mod_assign_testsuite

Moodle 3.2dev (Build: 20160817), 77350889716365104f5425e79cd2ef1e4e1e7e82

Php: 7.0.6, pgsql: 9.4beta3, OS: Darwin 15.5.0 x86_64

PHPUnit 5.4.8 by Sebastian Bergmann and contributors.

...............................................................  63 / 145 ( 43%)

............................................................... 126 / 145 ( 86%)

...................                                             145 / 145 (100%)

Time: 2.17 minutes, Memory: 112.30MB

OK (145 tests, 811 assertions)

That's approx 1 second per test. Many of those tests simply do not need that setup.