[master] Added user_chk hook library tests

Added tests for the example hooks library,
user_chk.

A new subdirectory was added under tests_v6:
    kea_only/user_chk

Which contains new tests which exercise the user_chk hook library:

New tests:
    v6.user_check.general.feature
    v6.user_check.vendor_options.feature

Data Files used by the above tests:
    outcome_1.txt
    outcome_2.txt
    registry_1.txt