Fix flaky ExistsQueryBuilderTests.testToQuery (#18995) (#19027)

---------

(cherry picked from commit 86ac3ab)

Signed-off-by: Atri Sharma <atri.jiit@gmail.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

server/src/test/java/org/opensearch/index/query/ExistsQueryBuilderTests.java

@@ -60,12 +60,20 @@ protected ExistsQueryBuilder doCreateTestQueryBuilder() {
         } else {
             fieldPattern = randomAlphaOfLengthBetween(1, 10);
         }
+
+        // Avoid patterns that would match the derived field "raw.derived_keyword"
+        // which doesn't support exists queries
+        if (fieldPattern.startsWith("raw")) {
+            fieldPattern = TEXT_FIELD_NAME;
+        }
+
         // also sometimes test wildcard patterns
         if (randomBoolean()) {
-            if (randomBoolean()) {
+            if (randomBoolean() && !fieldPattern.equals("r") && !fieldPattern.equals("ra")) {
                 fieldPattern = fieldPattern + "*";
             }
         }
+
         return new ExistsQueryBuilder(fieldPattern);
     }