Make WidgetTest::testBlockSettingsAreStoredCorrectly() less brittle

The test asserts content that has the same creation date in regards to the specific order in the view. That's brittle and seems to fail on more recent MySQL versions.

The order is not relevant for what we are testing here, it only matters whether or not the strings show up within the views output.

So I'm simplifying things to just use elementContains() and elementNotContains() with the CSS class of the view wrapper class, as a bonus this means we can also just use stark here as the theme for that test, making it more future-proof as well.