Fix cross-database test compatibility for immutable variables

Renamed test to run only on SQLite since json_extract() is SQLite-specific.
Other databases (PostgreSQL, MySQL, MSSQL) have different JSON functions.

Author: lovasoa

…t_files/it_works_immutable_variables.sql …variables_nomssql_nopostgres_nomysql.sqltests/sql_test_files/it_works_immutable_variables.sql renamed to tests/sql_test_files/it_works_immutable_variables_nomssql_nopostgres_nomysql.sql tests/sql_test_files/it_works_immutable_variables.sql renamed to tests/sql_test_files/it_works_immutable_variables_nomssql_nopostgres_nomysql.sql

@@ -1,3 +1,4 @@
+-- This test uses json_extract() which is SQLite-specific
 SET x = 'set_value';
 SET set_only = 'only_in_set';