mgr/dashboard: fix REST API test cases

mgr/dashboard: fix REST API test cases

1. Fix the test cases by using the 'assertJsonBody'
   method. The '_post' method doesn't return anything.
   That's why we need to use the 'assertJsonBody'
   method here instead of checking the fields directly.

2. Add the missing scope and permission definition of the
   'validate_password' resource. Otherwise the resource
   is not restricted and 'test_validate_password_invalid_permissions'
   will fail.

3. Re-word error messages. Change 'cannot' to 'must not'.

Signed-off-by: Tatjana Dehler <tdehler@suse.com>