Add testing of a "true" variant font

### What is the problem or limitation you are having?

As noted in #2159, the Endor font doesn't contain a "true" bold or italic, resulting in rendering on iOS and macOS that looks

This contrasts with GTK and Windows, which apply "fake" bold and italic:

### Describe the solution you'd like

* Add to the `fonts` example app an example of a "true" variant font. [Recursive](https://v-fonts.com/fonts/recursive) has been suggested as a candidate.
* Add a tests to the testbed canvas test that performs an automated test of what the `fonts` example is rendering. Add one test for each font (i.e. render an image that renders text as normal, bold, italic and bold-italic for each font).

### Describe alternatives you've considered

Don't explicitly test variant fonts.

### Additional context

_No response_

Start with the fonts example app and the testbed canvas test mentioned in the issue, then inspect how the existing fonts are rendered and tested. Add a true-variant font example and image-based coverage for normal, bold, italic, and bold-italic rendering for each font; done means the automated render tests verify the expected output.

Written by the indexing model from the issue text.