Extend tests of hywiki-display-referent-type

[matsl]

What

Extend tests of hywiki-display-referent-type

• test/hywiki-tests.el (hywiki-display-hywiki-test): Add helper
  display function.
  (hywiki-tests--display-referent-type): Verify right display function
  is called and hbut:current attributes are set.

Why

The call to the display function was not covered directly.

[rswgnu]

I'm thinking if you use a link-to-bookmark referent type, the display test verifies that the buffer and point match the ones linked to, and so on for all of the referent types.

[matsl]

I'm thinking if you use a link-to-bookmark referent type, the display test verifies that the buffer and point match the ones linked to, and so on for all of the referent types.

I'll respond on that tomorrow.

This PR is a general test that the function hywiki-display-referent-type handles error and builds the right display function plus setting the attributes. That does not have to be verified for each type. Not claiming that the test handles everything but is a good test of the function.