Tolerate lazy-loaded thumbnails in the image modal tests

[malberts]

MediaWiki master now defaults $wgNativeImageLazyLoading to true (false through REL1_46, changed for T148047), so thumbnails carry loading="lazy" and the image modal tests' thumbnail regexes fail on the MW master CI leg. Make the attribute optional, as the patterns already do for alt="" and /: loading is MediaWiki's markup, which this extension never emits, so the tests accept either form rather than pin the setting.

Considered, omitted:

• A MediaWiki version check: the attribute follows $wgNativeImageLazyLoading, not the version, so any setup that sets the flag explicitly would break it.
• Reading the flag to expect the attribute exactly when it is on: that would assert MediaWiki's behaviour rather than this extension's.
• The patterns still pin the rest of core's <img> attribute list, so a future core change can break them again; loosening them to what this extension controls is a larger rewrite.

_{AI-authored — Claude Code, Opus 5.5; follow-up proposed in session after @malberts flagged the red MW master leg, approach changed twice (optional group, then reading the flag, then back to the optional group after checking the suite's precedent); approach discussed in session with @malberts, no GitHub review yet; failure reproduced locally with the flag on, full unit suite run with it off, and the new patterns checked against MW master's actual output from CI.}