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.

Base css text, which could be shared by TemplateTestCase and page engine's
IframeTestCase, was moved to TestCase (to have a single source of it
without creating new dependencies / intermediary classes).

Close #586. Close #616.