test: ignore some warnings in test code (NFC)
test: ignore some warnings in test code (NFC)

Silence some diagnostics which clang now generates.  This makes it
easier to see the failures in lit output.  NFC.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@294871 91177308-0d34-0410-b5e6-96231b3b80d8

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