How to increment quantity column if record exists, else insert record?

I've got the below sqlite database:

CREATE TABLE `test` (
    `idx`   INTEGER PRIMARY KEY AUTOINCREMENT,
    `qty`   INTEGER,
    `brand` TEXT,
    `type`  TEXT
);

insert into test (qty, brand, type) values (1, "Merc", "petrol");
insert into test (qty, brand, type) values (1, "Toyota", "diesel");

I want to insert another record but if it exists then I want to increase the qty field, so:

insert into test (qty, brand, type) values (1, "Merc", "diesel");

so would just increase the qty field by one, and

insert into test (qty, brand, type) values (1, "Merc", "petrol");

would insert a new record.