False positive in shopify/jest/no-if

[BPScott]

One more edge case relating to #263.

It warns when the if is within a function, such as a react component you're defining within a test:

it('has a child with contentContext', () => {
    const Child: React.SFC = (_props, context) => {
      return context.withinContentContainer ? <div /> : null;
    };
}

As discovered in Shopify/polaris#1713

[BPScott]

I guess this will also stop https://github.com/Shopify/web/pull/14716 throwing a bunch of warnings when web updates to the newest SK too. (That PR changed ifs to ternarys but that loophole was recently closed in #298).