Improve automated tests

[straube]

I'm not sure whether all features can be tested outside a WP installation (possibly Mockery would be helpful on that) but at least the utilities and some other methods could be automatically tested with PHPUnit. I guess the first step to achieve that is moving the MultipleDomain class have to its own file. That would separate the plugin bootstrap logic from the class itself.

[straube]

A helpful link: https://make.wordpress.org/cli/handbook/plugin-unit-tests/. Perhaps?

[straube]

The tests we have now are too limited in terms of code coverage. They simply look for errors in instance creating. I'm reopening this issue to improve the tests.

[straube]

The current Travis configuration tests the code in old PHP releases. As an effort to make people use recent/secure versions, stop testing in these versions is a good thing.