Add automated tests

Problem/Motivation

As a module maintainer wanting to accept patches, I want to ensure that changes do not cause existing functionality to regress, so that I do not have to do a lot of manual testing.

Proposed resolution

Add automated tests.

Remaining tasks

1. Write automated tests.
2. Commit.
3. Set up regular automated branch and issue testing on Drupal.org.

User interface changes

None.

API changes

None.

Data model changes

None.