Add test for function property mutation

There were no previous test paths that tested it, so I'm adding the example from 
https://github.com/facebookexternal/forget-feedback/issues/32 as a fixture

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.mutate-function-property.expect.md

@@ -0,0 +1,21 @@
+
+## Input
+
+```javascript
+export function ViewModeSelector(props) {
+  const renderIcon = () => <AcceptIcon />;
+  renderIcon.displayName = "AcceptIcon";
+
+  return <Dropdown checkableIndicator={{ children: renderIcon }} />;
+}
+
+```
+
+
+## Error
+
+```
+[ReactForget] InvalidReact: This mutates a global or a variable after it was passed to React, which means that React cannot observe changes to it. (3:3)
+```
+          
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.mutate-function-property.js

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+export function ViewModeSelector(props) {
+  const renderIcon = () => <AcceptIcon />;
+  renderIcon.displayName = "AcceptIcon";
+
+  return <Dropdown checkableIndicator={{ children: renderIcon }} />;
+}