Add test to show inconsistency with jsdom.env

jsdom.env is normally used to be the easiest access point to parse a
website. However it doesn't load external scripts by default.
By supplying jsdom.env with the proper features config it is possible to
load external scripts, however, some bug in jsdom.env prevents that the
referenced external script is loaded an processed. If, however, a src or
scripts reference is supplied with the config, the referenced external
scripts on that page are also executed.
Add test for proposed API for env
Redefine API and edge cases better in new .env API test
Introduce create callback

The create callback will be called, as soon as the window for the
document is created, but before any scripts are executed in it. This
allows one to fix the window object with custom functions and missing
jsdom properties without patching jsdom.
Clearly define done and load call timing
Fix remaining timing/ordering issues.
Fix failing tests to be compatible with new api