[Impeller] Fix unused-result warnings.

[moko256]

Fix some compile errors due to the warnings error: ignoring return value of function declared with 'nodiscard' attribute [-Werror,-Wunused-result].
I found this when wireing the impeller's unittests for Windows.

List which issues are fixed by this PR. You must list at least one issue.

No changes in the flutter/tests repo.

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[cbracken]

Thanks for fixing!