Fix unused 'j' build failure
Fix unused 'j' build failure

Variable 'j' is unused in this test and clang emits a warning. `-Werror`
is enabled, so this warning is actually emitted as an error.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch