[FIX] web: mockServer and mockEnvironment

Fixes two issues in the test environment:

- for unset fields, the mockRead is supposed to return 0 (numerical
  fields), empty array (x2many fields) or false (otherwise). Except
  for the first case, it was returning undefined, and this might
  cause crashes when evaluating contexts or domains.
- the mock environment allows overriding some global properties
  like the session, for the test case only. Once the widget is
  destroyed, those global properties are reset to their original
  value. This wasn't working for models tests, because the destroy
  function of the parent widget was never called. This could make
  tests fail as they could have been impacted by other tests.