tests: disable dispatch_io on Windows for now

This test will require some work to port to Windows because it makes use
of <fts.h>. Disable it on Windows for now to make building easy.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index bc9930d..ba0d7af 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -169,8 +169,11 @@
        pingpong
        drift
        readsync
-       cascade
-       io)
+       cascade)
+  if(NOT WIN32)
+    # Not ported to Windows yet
+    list(APPEND DISPATCH_C_TESTS io)
+  endif()
   # an oddball; dispatch_priority.c compiled with -DUSE_SET_TARGET_QUEUE=1
   add_unit_test(dispatch_priority2 SOURCES dispatch_priority.c)
   target_compile_options(dispatch_priority2 PRIVATE -DUSE_SET_TARGET_QUEUE=1)