Make GDK specs environment fully isolated

What does this Merge Request do?

• Use gdk.example.yml in "test" mode instead of gdk.yml so we always have the same stable values
• Add GDK.root and use that instead of $gdk_root, so it's easy/safer to stub
• Default GDK.root to tmp folder so we don't accidentally mess-up with existing GDK instance

(This was extracted from previous work in !1020 (closed))

fix #860 (closed)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.