tests: fix test build
tests: fix test build

The DISPATCH_SIZEOF_PTR caused a test to go down the wrong path.  This
fixes the test and accounts for the Windows 64 bit case as well (which
is not a LP64 environment).

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