react: factor out assertCellValue
react: factor out assertCellValue

The motivation is twofold:
First, to reduce repetition in the code.
Second, to provide the user the ability to see the expected value of the
cell versus the observed value so the user has a better idea of what
went wrong.

exercism/xgo,robphoenix/exercism-go,exercism/xgo,robphoenix/exercism-go,leenipper/xgo,leenipper/xgo