LibTest+Spreadsheet: Add some basic spreadsheet runtime behaviour tests

As there's a somewhat active development going on, let's keep the
expected behaviour under tests to make sure nothing blows up :^)

Userland/Applications/Spreadsheet/Tests/test-harness.js

@@ -0,0 +1,46 @@
+describe("Harness-defined functions", () => {
+    test("createWorkbook", () => {
+        expect(createWorkbook).toBeDefined();
+        const workbook = createWorkbook();
+        expect(workbook).toBeDefined();
+        expect(workbook.sheet).toBeDefined();
+    });
+    test("createSheet", () => {
+        const workbook = createWorkbook();
+        const sheet = createSheet(workbook, "foo");
+        expect(sheet).toBeDefined();
+        expect(sheet.get_real_cell_contents).toBeDefined();
+        expect(sheet.set_real_cell_contents).toBeDefined();
+        expect(sheet.parse_cell_name).toBeDefined();
+        expect(sheet.current_cell_position).toBeDefined();
+        expect(sheet.column_index).toBeDefined();
+        expect(sheet.column_arithmetic).toBeDefined();
+        expect(sheet.get_column_bound).toBeDefined();
+    });
+    test("Sheet mock behavior", () => {
+        const workbook = createWorkbook();
+        const sheet = createSheet(workbook, "foo");
+        sheet.setCell("A", 0, "10");
+        expect(sheet.getCell("A", 0)).toEqual(["10", "10"]);
+
+        sheet.setCell("A", 0, "=10");
+        expect(sheet.getCell("A", 0)).toEqual(["=10", 10]);
+
+        expect(sheet.getColumns()).toEqual(["A"]);
+    });
+    test("Workbook mock behavior", () => {
+        const workbook = createWorkbook();
+        const sheet = createSheet(workbook, "foo");
+        expect(workbook.sheet("foo")).toBe(sheet);
+        expect(workbook.sheet(0)).toBe(sheet);
+        expect(workbook.sheet(1)).toBeUndefined();
+        expect(workbook.sheet("bar")).toBeUndefined();
+    });
+    test("Referencing cells", () => {
+        const workbook = createWorkbook();
+        const sheet = createSheet(workbook, "foo");
+        sheet.setCell("A", 0, "42");
+        sheet.setCell("A", 1, "=A0");
+        expect(sheet.getCell("A", 1)).toEqual(["=A0", "42"]);
+    });
+});