testing: separate the posix import into target files

This is needed for the docs generator to generate all the docs for the
posix package, if it is imported like it was on Windows it would
generate docs for the Windows version of the package which has much less
symbols exposed.

core/testing/signal_handler_windows.odin

@@ -0,0 +1,6 @@
+#+private
+package testing
+
+__setup_signal_handler :: proc() {}
+
+_test_thread_cancel :: proc "contextless" () {}