Add setup for database_cleaner in core

* Cleanup test DB prior each suite run for reproducible results
Add optional database cleaning transaction around examples
Fix code style to prepare change
Add db isolation for Spree::OrderContents specs

* This is needed for determinism and reproducible results
* Also allows conncurrent execution for parallel mutation tests
* One step into randomizing spec order
Update mutant dependency
Fix coverage of OrderContents#add{,_to_line_item}
Fix adjusting non valid line items