Tests: Restore suppression of xcodebuild error with Xcode Command-Line Tools

In commit 40fa2c1c90 (Tests: Cleanup CMake_TEST_XCODE_VERSION code,
2024-04-29, v3.30.0-rc1~183^2) we accidentally stopped capturing the
stderr from `xcodebuild`, causing it to appear in cmake's output.
The error is incidental and tolerated, so do not let the user see it.