Fixes + Test for "Get base url from base-url data param"

Fixes tests and add one for the change in
https://github.com/plone/mockup/commit/919f94339b23d3fa55d642d50257f9807ef1cd4f
by @ACatlla
Even Plone 5 dropped the base tag, we should still support base tags.
The regex is needed, as jQuery unreliably parses HTML strings.