test(template-require-valid-alt-text): absorb audit-fixture cases, drop audit fixture

Upstream maintainers don't want the per-PR `tests/audit/peer-parity`
pattern. Port one case that pinned distinct behavior:
- `<img alt=" " />` as VALID — whitespace-only alt is currently
  treated as decorative; jsx-a11y agrees.

All other audit cases were already covered by the regular tests on
this branch (extensive existing coverage of object/area/input variants,
empty aria-label/labelledby, presentation-role conflicts, etc).

Author: johanrd

tests/audit/alt-text/peer-parity.js

@@ -12,6 +12,9 @@ ruleTester.run('template-require-valid-alt-text', rule, {
     '<template><img alt="Company branding" src="/logo.png" /></template>',
     '<template><img alt="" src="/decorative.png" /></template>',
     '<template><img hidden alt="" /></template>',
+    // Whitespace-only alt — pin our current behavior. Peer plugins
+    // (jsx-a11y) accept this; we don't trim before considering "empty alt".
+    '<template><img alt=" " /></template>',
 
     '<template><img alt="hullo"></template>',
     '<template><img alt={{foo}}></template>',