Issue #204: Move all tests for core modules into /tests folder.

PR for https://github.com/backdrop/backdrop-issues/issues/204. A few things to note:

- I moved all .test files in the core modules into a /tests folder
- I moved (almost) all test modules into a subfolder inside the /tests folder. The only exception are the test modules in the simpletest folder. I left those because it was really complicated. :sob: I can take another crack at it if you think it would be better for this PR to include that, too.
- I am not sure that <code>core/modules/locale/tests/locale_test/test.xx.po</code> should be in the submodule folder, but all the locale tests are passing, so I left it there.
- There are some references in <code>core/modules/update/tests/update.test</code> to modules with version 8.x-1.0, etc (see lines 315, 359, 366, etc). I left those alone, but I'm not sure if they should be changed in another issue.