Squish Tests: Can remove block from group

As a developer I would like to verify the following behaviour using Squish:

- Create a block, create a group and add the block to it.
- Remove the block from the group.
- Verify that the block has been removed from the group.

#### Additional info:
Consider refactoring `tst_can_add_block_to_a_group` to reduce duplication as we are adding a block to a group anyways.