test: update axe tests to expect image-alt instead of link-name

The link-name violation from the slide-menu button is now fixed,
so revealjs axe tests need a different violation to detect. Add
an unlabeled img to the console and json fixtures (matching the
document fixtures) and switch expectedViolation to image-alt.

Author: mcanouil

tests/docs/playwright/revealjs/axe-console.qmd

@@ -13,3 +13,5 @@ This text violates contrast rules: [insufficient contrast.]{style="color: #eee;
 ## Slide 2
 
 More content to check.
+
+<img src="placeholder.png" class="axe-test-img">

tests/docs/playwright/revealjs/axe-json.qmd

@@ -14,3 +14,5 @@ This text violates contrast rules: [insufficient contrast.]{style="color: #eee;
 ## Slide 2
 
 More content to check.
+
+<img src="placeholder.png" class="axe-test-img">

tests/integration/playwright/tests/axe-accessibility.spec.ts

@@ -50,18 +50,18 @@ const testCases: AxeTestCase[] = [
 
   // RevealJS — axe-check.js loads as standalone module (#13781).
   // RevealJS CSS transforms prevent axe-core from computing color contrast,
-  // so we check for link-name (slide-menu-button has unlabeled <a>).
+  // so we check for image-alt (unlabeled <img> on Slide 2).
   { format: 'revealjs', outputMode: 'document', url: '/revealjs/axe-accessibility.html',
-    expectedViolation: 'link-name' },
+    expectedViolation: 'image-alt' },
   { format: 'revealjs', outputMode: 'console', url: '/revealjs/axe-console.html',
-    expectedViolation: 'link-name' },
+    expectedViolation: 'image-alt' },
   { format: 'revealjs', outputMode: 'json', url: '/revealjs/axe-json.html',
-    expectedViolation: 'link-name' },
+    expectedViolation: 'image-alt' },
 
   // RevealJS dark theme — verifies CSS custom property bridge for theming.
   // Report should use --r-background-color/#191919, not the Sass fallback #fff.
   { format: 'revealjs-dark', outputMode: 'document', url: '/revealjs/axe-accessibility-dark.html',
-    expectedViolation: 'link-name' },
+    expectedViolation: 'image-alt' },
 
   // Dashboard — axe-check.js loads as standalone module, falls back to document.body (#13781)
   { format: 'dashboard', outputMode: 'document', url: '/dashboard/axe-accessibility.html',