silence new -Wunused-result warnings in test

No functional changes intended.

After https://github.com/llvm/llvm-project/commit/f156b51aecc676a9051136f6f5cb74e37dd574d1,
new -Wunused-result warnings popped up in this test:
https://buildkite.com/llvm-project/upstream-bazel/builds/28320#bc3ec049-af39-4114-b7b8-4cbc180bc09b