[Tests] Fix rare edge case in SimpleQueryStringBuilderTests (#35201)

If the random query string is "now" by accident _and_ we are also not setting
some field names to use explicitely, then we can hit the "mapped_date" field
from default test setup. This correctly leads to the query being was marked as
not cacheable, but we assume and check so later. This change fixes this rare
edge case by making sure we don't hit the "date" field in this rare cases.

Closes #35183

server/src/test/java/org/elasticsearch/index/query/SimpleQueryStringBuilderTests.java

@@ -105,6 +105,12 @@ protected SimpleQueryStringBuilder doCreateTestQueryBuilder() {
                 fields.put(STRING_FIELD_NAME_2, 2.0f / randomIntBetween(1, 20));
             }
         }
+        // special handling if query is "now" and no field specified. This hits the "mapped_date" field which leads to the query not being
+        // cacheable and trigger later test failures (see https://github.com/elastic/elasticsearch/issues/35183)
+        if (fieldCount == 0 && result.value().equalsIgnoreCase("now")) {
+            fields.put(STRING_FIELD_NAME_2, 2.0f / randomIntBetween(1, 20));
+        }
+
         result.fields(fields);
         if (randomBoolean()) {
             result.autoGenerateSynonymsPhraseQuery(randomBoolean());