Summary:
CI mode prevents Jest from automatically writing snapshots, and also
causes obsolete snapshots to be an error instead of a warning. This is
consistent with the behavior on Travis, where the `CI=1` environment
variable is set. It should thus be the default when running `yarn test`
(but not `yarn unit`).
Test Plan:
Add a file `src/foo.test.js`:
```js
// @flow
describe("foo", () => {
it("bar", () => {
expect("baz").toMatchSnapshot();
});
});
```
Note that `yarn test` fails with the message, “new snapshot was not
written”.
Revert this change, then re-run `yarn test`; note that it passes,
writing a snapshot.
Then, reinstate this change and delete `src/foo.test.js`. Note that
running `yarn test` fails, due to an obsolete snapshot. Revert the
change again, and watch `yarn test` pass despite the obsolete snapshot.
Finally, remove the snapshot. :-)
wchargin-branch: test-jest-ci
4b0693e2a7