nixos/tests: move mysql tests to subfolder

nixos/tests/mysql/testdb.sql

@@ -0,0 +1,11 @@
+create table tests
+( Id   INTEGER      NOT NULL,
+  Name VARCHAR(255) NOT NULL,
+  primary key(Id)
+);
+
+insert into tests values (1, 'a');
+insert into tests values (2, 'b');
+insert into tests values (3, 'c');
+insert into tests values (4, 'd');
+insert into tests values (5, 'hello');