Joomla! CMS is quite famous as a system written in PHP. But still there are certain things that PHP alone would not be able to address. Being a server side scripting language, PHP does not have any authority in what happens on the client side. Therefore PHP becomes helpless in doing even very simple things that involve changing the web page content depending on user’s input.
Now assume we had a comprehensive test suite covering the validate.js functionality. The moment the Mootools removal commit is made, Travis would start running the tests against the Pull Request and would go complaining about the test cases being failed. That way the person, who made the Pull Request is going to identify what mistakes she or he made and what needs to be done to fix it. If this was the case, the issue #10259 would not have even showed up and a lot of time and effort of multiple contributors could have been saved.
Every pull request in Joomla needs to be tested successfully by at least two people before it can be merged. In most issues, it can be well observed that multiple iterations of fixing and testing is needed to be carried out until the issue gets completely fixed. https://issues.joomla.org/tracker/joomla-cms/9243 is an example for that. This happens due to two reasons. One is the inability of a single person to look and try out all the possible ways his or her fix could be tested. The other reason is the fact that there is a high possibility an outsider might notice something the developer missed. But still this process consumes a significant amount of time and sometimes the problems they find are only the pretty straightforward ones. Having a comprehensive test suite would definitely reduce a considerable amount of this time.
Please don’t forget to test and try it out and provide any feedback you have as well :) Thank you!