Implement tests where needed

[acicovic]

Is your feature request related to a problem?

We strive to write tests for all new code that is added to the plugin. Due to workload, sometimes tests come after the integration of the code.

This thread is:

• A catch-all to track where tests need to be implemented for recent code.
• A call to action to examine what tests should be written for older code that is of high impact or prone to breakages.

Describe the solution you'd like

1. Write tests (unit, integration, E2E or combination) where applicable for recent code.
2. If resources allow it, expand this effort to older code that is of high impact or prone to breakages.

Recent code that has no or needs more tests

• [ ] https://github.com/Parsely/wp-parsely/issues/986.
• [ ] https://github.com/Parsely/wp-parsely/pull/1159.
• [ ] PARTIALLY DONE - After a bug which came from a missing require (which also resulted in our implementation of autoloading), we noticed that our metadata Tag_Builder and Date_Builder classes have no tests.
• [ ] The Dashboard Widget needs to have tests implemented (We've added integration tests here as part of #1539.
• [ ] https://github.com/Parsely/wp-parsely/pull/1784#discussion_r1294436552
• [ ] https://github.com/Parsely/wp-parsely/pull/1772#discussion_r1294434065
• [ ] https://github.com/Parsely/wp-parsely/pull/1789#discussion_r1298124726

Other related issues

• [x] DONE - https://github.com/Parsely/wp-parsely/issues/1121