[DRAFT] Generalize unit tests to also allow use against REST CI backend

[eddelbuettel]

This (currently draft) PR regroups slightly altered tests which have been updated to also allow runing against REST CI unit tests. The setup in the (private to TileDB) repo TileDB-REST-CI permits staging a REST instance for CI use. (We currently rely on a lighter-weight shell script to launch components.)

The plan is to run these tests nightly as an a scheduled action. The PR will likely remain in draft until all pieces are in place.

The main change to tests is fairly straightforward and replaces base::tempfile() -- which generates filenames such as /tmp/RtmpBbAPzz/file3b526c17a67c7e where /tmp is a default overrideable by environment variable TMP, TEMP or TEMPDIR and is and the second value, the temporary directory of the current R session, are provided by argument tmpdir which defaults to base::tempdir(). We override this with a local copy setting tmpdir to tiledb://unit which directs the TileDB Core API to route this to the configured REST server with enabled namespace unit.

A few tests are skipped, this is expected and known: VFD, file listing, ... are not applicable for REST. The vast majority however is, and those tests run against the REST CI. (For completeness, a few unit tests are also 'file-less': tests of dimension, domain, schema, ... when these are merely created and not materialized. We still run them because there is little value is added complexity to skip between 'REST' and 'non-REST' tests.

SC 38763

[shortcut-integration]

This pull request has been linked to Shortcut Story #38763: Generalize tests to allow use in REST CI tests.