test: add ignore utility test (#25891)

lib/util/ignore.spec.ts

+import { logger } from '../logger';
+import { isSkipComment } from './ignore';
+
+jest.mock('../logger', () => ({
+  logger: {
+    debug: jest.fn(),
+  },
+}));
+
+describe('util/ignore', () => {
+  it('returns true for "renovate:ignore" comments', () => {
+    expect(isSkipComment('renovate:ignore')).toBe(true);
+  });
+
+  it('returns false for comments not starting with "renovate:" or "pyup:"', () => {
+    expect(isSkipComment('other:ignore')).toBe(false);
+  });
+
+  it('returns false for "renovate:" comments without "ignore"', () => {
+    expect(isSkipComment('renovate:update')).toBe(false);
+  });
+
+  it('logs unknown command for "renovate:" comments without "ignore"', () => {
+    isSkipComment('renovate:update');
+    expect(logger.debug).toHaveBeenCalledWith(
+      'Unknown comment command: update',
+    );
+  });
+
+  it('returns false when comment is undefined', () => {
+    expect(isSkipComment()).toBe(false);
+  });
+});