improved test coverage with findRenderedComponentWithType test

[dustybins]

Summary

I wanted make my first contribution to an open source project by adding a simple test to improve test coverage.
ReactTestUtils.js was missing coverage on lines 292-294.

Test Plan

Added a new test to verify that the findRenderedComponentWithType method was indeed finding the rendered component with the given type.

yarn test reactutils-test --coverage
Was able to compare test results and verify that the test was passed in addition to improving test coverage.

[sizebot]

Comparing: af1a4cb...fc2f00b

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	122.73 kB	122.73 kB	=	39.39 kB	39.39 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	129.31 kB	129.31 kB	=	41.48 kB	41.48 kB
facebook-www/ReactDOM-prod.classic.js	=	412.24 kB	412.24 kB	=	76.24 kB	76.24 kB
facebook-www/ReactDOM-prod.modern.js	=	400.30 kB	400.30 kB	=	74.33 kB	74.33 kB
facebook-www/ReactDOMForked-prod.classic.js	=	412.24 kB	412.24 kB	=	76.24 kB	76.24 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against fc2f00b

[stale]

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