test: route rlike non-scalar-pattern fallback test through engine=rust

The per-expression spark.comet.expression.regexp.allowIncompatible flag
is no longer consulted by the regex family. Switch to engine=rust so the
RLike serde reaches convertViaNativeRegex and emits the
'Only scalar regexp patterns are supported' fallback message the test
asserts on.

Author: andygrove

spark/src/test/scala/org/apache/comet/CometExpressionSuite.scala

@@ -967,7 +967,7 @@ class CometExpressionSuite extends CometTestBase with AdaptiveSparkPlanHelper {
     withTable(table) {
       sql(s"create table $table(id int, name varchar(20)) using parquet")
       sql(s"insert into $table values(1,'James Smith')")
-      withSQLConf(CometConf.getExprAllowIncompatConfigKey("regexp") -> "true") {
+      withSQLConf(CometConf.COMET_REGEXP_ENGINE.key -> CometConf.REGEXP_ENGINE_RUST) {
         val query2 = sql(s"select id from $table where name rlike name")
         val (_, cometPlan) = checkSparkAnswer(query2)
         val explain = new ExtendedExplainInfo().generateExtendedInfo(cometPlan)