refactor env settings for template tests

Split cssText from iframe tests cfg into baseCssText and
additionalCssText. Thanks to this, one may just add a bunch of overriden /
new css properties for a specific test, instead of having to copy-paste
everything.

----

I know it's not probably the most "clean" to move the CSS_TEXT to `TestCase.js` but I don't want to create new classes or make page engine's `IframeTestCase` depend on `TemplateTestCase`.