[test](case) rename session var disable_variant_flatten_nested to enable_variant_flatten_nested (#54931)

Author: XueYuhai

regression-test/suites/variant_github_events_nonConcurrent_upgrade_p2/load.groovy

@@ -23,7 +23,7 @@ suite("regression_test_variant_github_events_upgrade_p2", "nonConcurrent,p2"){
     def delta_time = 1000
     def alter_res = "null"
     def useTime = 0
-    sql "set disable_variant_flatten_nested = false"
+    sql "set enable_variant_flatten_nested = true"
 
     def wait_for_latest_op_on_table_finish = { table_name, OpTimeout ->
         for(int t = delta_time; t <= OpTimeout; t += delta_time){
@@ -73,7 +73,7 @@ suite("regression_test_variant_github_events_upgrade_p2", "nonConcurrent,p2"){
     }
 
     def table_name = "github_events"
-    sql """DROP TABLE IF EXISTS ${table_name}"""
+//    sql """DROP TABLE IF EXISTS ${table_name}"""
     table_name = "github_events"
     sql """
         CREATE TABLE IF NOT EXISTS ${table_name} (

regression-test/suites/variant_p2/tpch_upgrade/load.groovy

@@ -36,7 +36,7 @@ suite("load") {
                   "supplier"]
 
     tables.forEach { tableName ->
-        sql "DROP TABLE IF EXISTS ${tableName}"
+//        sql "DROP TABLE IF EXISTS ${tableName}"
         sql """
                 CREATE TABLE IF NOT EXISTS ${tableName} (
                     k bigint,