…t_strict**.

At most assert it is already set using is_strict.  Since set_strict mutates the settings-object.

Also:
 - Caches get_sp_cert and get_sp_key (since these can come from the file-system).

- Since not all "memory-loading" C functions are exposed in the dm.xmlsec.binding; cache the
  add_sign signing_key (only used for generating SP metadata).

- Manipulate onelogin.saml2.time_indirect `mocked_time_fn'

    Set it to a function that returns a time.time()-like float;
    when reset to a falsy-value mocking can be turned off again.

- Added test-context-managers
- Implementing tests that mocking works.