Add tests that tests blocks with multiple heads

[islamaliev]

At the moment we don't have any test that tests merging a block with multiple heads. If we pick the first head all tests will pass.
The relevant code is in internal/db/merge.go in func (mp *mergeProcessor) loadBlocks.

This is sensitive and crucial part of defradb and should be properly tested.

This will be covered anyway once #2769 is in place, but it's encryption-related and we might want to have a separate test focused purely on this part.