Jest

Jest takes the transform through @plumeria/swc-jest. It compiles the styling prop away, then hands the file to @swc/jest.

npm i -D @plumeria/swc-jest @swc/core

Put it where @swc/jest was. Jest runs one transformer per pattern, so it replaces the entry rather than sitting beside it:

module.exports = {
  testEnvironment: 'jsdom',
  transform: {
    '^.+\\.(t|j)sx?$': '@plumeria/swc-jest',
  },
};

SWC is configured as before, from .swcrc or from the options next to the transformer. A project on ts-jest, babel-jest or next/jest moves that entry to @plumeria/swc-jest; type checking then belongs to tsc, not to the test run.

Do

Assert what the component does. The class names are real, so a branch that picks a different style shows up as a different className:

import { render, screen } from '@testing-library/react';
import { Tabs } from './tabs';

test('the active tab is styled apart from the rest', () => {
  render(<Tabs active="b" />);

  const a = screen.getByRole('tab', { name: 'A' });
  const b = screen.getByRole('tab', { name: 'B' });

  expect(b.getAttribute('aria-selected')).toBe('true');
  expect(b.className).not.toBe(a.className);
});

• Test text, roles, state and events, as you would without Plumeria.
• Compare class names between two renders or two elements to check a conditional style took effect.
• Import styles from other files freely. They compile as in the build, and Jest's cache follows them: change a shared style and the files that use it are compiled again.

Don't

expect(button.className).toBe('xqqbxt1d xq96bg3w'); // don't
expect(getComputedStyle(button).color).toBe('red'); // don't

• Don't hard-code a generated class name. Each one is a property–value pair, hashed: add a property to the style and the test fails, having found nothing wrong with the component.
• Don't assert computed styles. No stylesheet is written, and jsdom applies none; see What jsdom does not give you.
• Don't list it beside @swc/jest, ts-jest or babel-jest for the same files. Only one transformer runs per pattern.
• Don't test what a style compiles to here. Call the transform directly; see Driving the transform.