test/unit/merge: Better add & update file coverage
test/unit/merge: Better add & update file coverage

  We add more `Merge.addFileAsync()` tests to have better coverage of
  conflicting situations (i.e. a record with the same path already
  exists in PouchDB) and refactor some `Merge.updateFileAsync()` tests
  to better reflect what is being tested and the reality of records that
  are involved in such situations.

cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop