Function backup

test/36.database.backup.js:139–139

	});
	it('should be aborted if the progress callback returns a non-number', async function () {
		const backup = x => this.db.backup(util.next(), { progress: () => x });
		await rejectsWith(TypeError, backup(null));
		await rejectsWith(TypeError, backup(new Number(1)));
		await rejectsWith(TypeError, backup(() => 1));