Add a test for Buffer flush before shutdown

tests/queries/0_stateless/01870_buffer_flush.reference

@@ -0,0 +1,2 @@
+0
+5

tests/queries/0_stateless/01870_buffer_flush.sql

@@ -0,0 +1,26 @@
+-- Check that Buffer will be flushed before shutdown
+-- (via DETACH DATABASE)
+
+drop database if exists db_01870;
+create database db_01870;
+
+-- Right now the order for shutdown is defined and it is:
+-- (prefixes are important, to define the order)
+-- - a_data_01870
+-- - z_buffer_01870
+-- so on DETACH DATABASE the following error will be printed:
+--
+--     Destination table default.a_data_01870 doesn't exist. Block of data is discarded.
+create table db_01870.a_data_01870 as system.numbers Engine=TinyLog();
+create table db_01870.z_buffer_01870 as system.numbers Engine=Buffer(db_01870, a_data_01870, 1,
+    100, 100, /* time */
+    100, 100, /* rows */
+    100, 1e6  /* bytes */
+);
+insert into db_01870.z_buffer_01870 select * from system.numbers limit 5;
+select count() from db_01870.a_data_01870;
+detach database db_01870;
+attach database db_01870;
+select count() from db_01870.a_data_01870;
+
+drop database db_01870;

tests/queries/skip_list.json

@@ -720,6 +720,7 @@
         "01804_dictionary_decimal256_type",
         "01850_dist_INSERT_preserve_error", // uses cluster with different static databases shard_0/shard_1
         "01821_table_comment",
-        "01710_projection_fetch"
+        "01710_projection_fetch",
+        "01870_buffer_flush" // creates database
     ]
 }