[addon-test] Expose the assert API from Vitest

[adriantrunzo]

Is your feature request related to a problem? Please describe.

The assert API is very handy in tests, for example when asserting that a variable is defined and having TS be able to use the assertion result.

Describe the solution you'd like

I'd like to be able to use import { assert } from "@storybook/test";

Describe alternatives you've considered

I can define my own invariant function like:

export function invariant(
  condition: unknown,
  message: string,
): asserts condition {
  if (!condition) {
    throw new Error(message);
  }
}

But this functionality is already built into Vitest so it would be great to have support for assert out of the box.

Are you able to assist to bring the feature to reality?

yes, I can

Additional context

No response