4040: update FUSE driver tests with stable fixtures

Added test fixtures specifically for FUSE driver tests.
4040: clarify variable names

Per code review: give d1, d2, d3 more descriptive variable names.
4040: code review feedback

* Renamed
** collection_owned_by_fuse_1 -> collection_1_owned_by_fuse
** collection_owned_by_fuse_2 -> collection_2_owned_by_fuse
* Added fixtures pipeline_instance_owned_by_fuse and
  pipeline_instance_in_fuse_project.
* Added name link for the empty collection, to exercise that links are
  exposed in the filesystem in the expected way.
Merge branch '4040-fuse-tests'

Fixes #4040.