feat: add mock function

src/Test.res

@@ -1,6 +1,24 @@
 open Types
 // TODO: mocking etc
 
+/**
+   * Create mocks with the `mock` function.
+   *
+   * ```ts
+   * import { test, expect, mock } from "bun:test";
+   * const random = mock(() => Math.random());
+   *
+   * test("random", async () => {
+   * const val = random();
+   * expect(val).toBeGreaterThan(0);
+   * expect(random).toHaveBeenCalled();
+   * expect(random).toHaveBeenCalledTimes(1);
+   * });
+   * ```
+   */
+@module("bun:test")
+external mock: (unit => 'value) => unit => 'value = "mock"
+
 /**
    * Control the system time used by:
    * - `Date.now()`

test/Mock.test.res

@@ -0,0 +1,15 @@
+open Test
+
+let mockFunction = mock(() => ())
+
+describe("mock", () => {
+  beforeAll(() => {
+    mockFunction()
+  })
+
+  test("mock function should have been called", () => {
+    expect(mockFunction->Obj.magic)->Expect.toHaveBeenCalled
+    mockFunction()
+    expect(mockFunction->Obj.magic)->Expect.toHaveBeenCalledTimes(2.)
+  })
+})