cmd/screentest: add test cases
cmd/screentest: add test cases

Change-Id: Ia9bd2f814c56b8f6d24b37b5a045fd3a31d05892
Reviewed-on: https://go-review.googlesource.com/c/website/+/374775
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Trust: Jamal Carvalho <19dd88ea049b99d7eb75a23cc7da743ca525bca4@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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