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 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