Caption shortcode shouldn't output an invalid HTML ID

[helen]

While looking more closely at core shortcodes post-4.2.3, I noticed that the caption one escapes the provided ID and is safe, but that doesn't mean it's necessarily going to produce a valid HTML ID. We should also use sanitize_html_class(), since that's what it's for.

[helen]

In 33489:

Ensure the caption shortcode outputs a valid HTML ID.

fixes #33179.

[helen]

In 33506:

Fix a unit test failure after [33489].

see #33179.