I decided the best solution to this problem was to refresh the project workspace but this solution didn't get to the bottom of the issue why the tests were not running in Intellij.

The steps I took were:

1. Checked in my code changes into the project on GITHub.
2. Deleted the project from my workspace.
3. Restored the project workspace from the GITHub.
4. Checked out the project branch I was working on.

This has resolved the problem and Intellij tests now run.