Issue 109 web testing

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [#109]
| License       | MIT
| Doc PR        | []

This is the admin part of the block bundle web tests.

As asked I centralized the code in AbstractBlockAdminTestCase and refactor david simpleblock test case to fit it.

I didn't do the Image block as I think it should go with #38 and #72