[regexp] Document incorrect assertion in intl/regress-10573.js

There are at least three equivalence classes where this assertion
should not actually hold:

  '\u0390\u1fd3',              // ΐΐ
  '\u03b0\u1fe3',              // ΰΰ
  '\ufb05\ufb06',              // ſtst

Bug: v8:10591
Change-Id: I26cb43d2e67c54e689f1831ea13be46c73d5e92d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2231595Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68246}

test/intl/regress-10573.js

@@ -26,6 +26,9 @@ function TestEquivalenceClass(eclass) {
       var shouldMatch = Canonicalize(c1) === Canonicalize(c2);
 
       assertEquals(backref.test(cc), shouldMatch);
+
+      //TODO(v8:10591): Update expectations for ΐΐ, ΰΰ, and ſtst once
+      //case folding is fixed.
       assertEquals(backrefUnicode.test(cc), true);
     }
   }