Add mockRandom utility for testing

assets/test/mock-date-now.ts

@@ -1,9 +0,0 @@
-export function mockDateNow(initialDateNow: number): void {
-  beforeAll(() => {
-    vi.useFakeTimers().setSystemTime(initialDateNow);
-  });
-
-  afterAll(() => {
-    vi.useRealTimers();
-  });
-}

assets/test/mock.ts

@@ -0,0 +1,18 @@
+export function mockDateNow(initialDateNow: number): void {
+  beforeAll(() => {
+    vi.useFakeTimers().setSystemTime(initialDateNow);
+  });
+
+  afterAll(() => {
+    vi.useRealTimers();
+  });
+}
+
+/**
+ * Mocks `Math.random` to return a static value.
+ */
+export function mockRandom(staticValue = 0.5) {
+  const realRandom = Math.random;
+  beforeEach(() => (Math.random = () => staticValue));
+  afterEach(() => (Math.random = realRandom));
+}