add a test to count the checkboxes

src/components/__tests__/MthGrid.spec.ts

@@ -111,6 +111,8 @@ describe("MthGrid.vue ------ display global elements", () => {
     expect(wrapper.find("table").exists()).toBe(true);
     expect(wrapper.findAll("th").length).equals(headers.length + 1); // checkbox column
     expect(wrapper.findAll("tbody > tr").length).equals(items.length);
+    // count checkboxes
+    expect(wrapper.findAll("input[type='checkbox']").length).equals(items.length + 1); // select all
   });
 });