Adapt getBorrowingProcessByUserId() & deleteBorrowingProcess() to return an array with borrowingProcesses

Change BeforeEach, disable testDeleteRequestDeletingABookIsSuccessful() and testAddBookAndUserToBorrowingProcess(), add testDeleteBorrowingProcessRequestIsSuccessful() and testIfUserIsDeletedWhenSendingDeleteBorrowingProcessRequest()

Add testDeleteBorrowingProcessIsSuccessful, testDeleteBorrowingProcessIsNotSuccessfulWithExistingBook and change to MockitoJUnitRunner

Add deleting borrowingProcess method with null check and change the return type of getBorrowingProcessByUserId()

Add query book and user object by id in registerBorrowingProcess, change getBorrowingProcessByUserId to get the borrowingProcess by userId

Add testCreateBorrowingProcessWithBookAndUser, testAddBookAndUserToBorrowingProcess and reformat code lines

Change request parameter in post register borrowingProcess, adapt method to dto borrowingProcessCreationRequest

Adapt addBookToBorrowingProcess() to create a borrowingProcess, set relations between user, book and borrowingProcess and save it

Add JsonBackReference annotation to handle infinite recursion of json serialization and change possible character length to add description to longtext

Change return values of controller methods to ResponseEntity, adapt tests and change crossorigin annotations position