Use Bioconductor longtests

[pkrog]

• See https://bioconductor.org/developers/how-to/long-tests/.
• [x] Put a longtests folder at the package's root, with the same structure as the tests folder.
• [x] Add a .BBSoptions file at the package's root containing RunLongTests: TRUE.
• [x] Use a inst/testref folder to put all JSON entry ref files.
• [x] Modify makefile to run the tests from this folder.
• [x] Modify ExtTests to generate long tests with option longtests=TRUE, inside this new folder and with the .BBSoptions file.