How to programmatically set an active context

Changed setActiveContext($name) to setActiveContexts($context_names);
Added unsetActiveContexts($context_names);
Added isActiveContext($name);

The changes worked for me. I'm just wondering if maybe there should be some tests made for this code because it's not used by any other code and someone might potentially delete it in the future. So I'll leave it as Needs Review for now, just so people can debate this a bit.

I have submitted a commit that removes an unnecessary newline in the ContextManager.php file.
Additionally, I have written basic kernel tests to validate the new functionality implemented in this issue.

Thanks for the test @agile-mark-l this looks good.