fix/add tests for validation and ISO 8601

[seth-shaw-asu]

GitHub Issue: #103

What does this Pull Request do?

Addresses issue with date intervals throwing a warning when passed to EDTF:Utils::iso8601Value. Also updates and adds more tests.

• Does this change require documentation to be updated? No.
• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? No.
• Could this change impact execution of existing code? No.

How should this be tested?

Short version: tests pass. 👍

Longer version:

• Spin up a site with controlled_access_terms.
• using the command-line: drush ev 'print(\Drupal\controlled_access_terms\EDTFUtils::iso8601Value("2023-07/2023-08")."\n");'
• Observe the warning.
• Apply PR
• Run the command again and no warning appears.

Additional Notes

An early version of this was accidentally pushed directly to the repo which I then reverted. This PR is the real version.

Interested parties

@Islandora/committers

[alxp]

Tested the drush command and phpunit on Drupal 10, works as advertised.