Linking a test to multiple features

[foolip]

This came up in #45475 (review).

The syntax of WEB_FEATURES.yml makes it easy to link one test to multiple features, but I'm a bit scared to do it because it won't be at all obvious if it's done purposefully or by having accidentally overlapping filename patterns.

Should we require something more explicit yo link a test to multiple features, so that it doesn't happen by accident?

cc @jcscottiii