[FIX] web: prevent test interference

In the view dialog test suite, a few tests created a parent widget, then
created a view dialog, and destroyed the view dialog at the end, but not
the parent widget.  As a result, the environment was not properly
restored at the end.

In this case, the test 'SelectCreateDialog correctly evaluates domains'
changed the session (with user_context: {uid: 2}), and this made other
tests crash (web_editor test in particular)