ESQL: Fix case-insensitive test generation with Unicodes (elastic#128753)

This excludes from testing the strings containing Unicode chars that
change length when changing case.

Closes elastic#128705 Closes elastic#128706 Closes elastic#128710 Closes elastic#128711 Closes
Closes elastic#128717 Closes elastic#128789 Closes elastic#128790 Closes elastic#128791 Closes

(cherry picked from commit 092d4ba)

Author: bpintea

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/string/RLikeTests.java

@@ -94,8 +94,15 @@ private static void casesForString(
             return new TextAndPattern(text, escapeString.apply(text));
         }, true);
         cases(cases, title + " matches self case insensitive", () -> {
-            String text = textSupplier.get();
-            return new TextAndPattern(randomCasing(text), escapeString.apply(text));
+            // RegExp doesn't support case-insensitive matching for Unicodes whose length changes when the case changes.
+            // Example: a case-insensitive ES regexp query for the pattern `weiß` won't match the value `WEISS` (but will match `WEIß`).
+            // Or `ʼn` (U+0149) vs. `ʼN` (U+02BC U+004E).
+            String text, caseChanged;
+            for (text = textSupplier.get(), caseChanged = randomCasing(text); text.length() != caseChanged.length();) {
+                text = textSupplier.get();
+                caseChanged = randomCasing(text);
+            }
+            return new TextAndPattern(caseChanged, escapeString.apply(text));
         }, true, true);
         cases(cases, title + " doesn't match self with trailing", () -> {
             String text = textSupplier.get();