Fix race condition in TestIOWait#test_wait

Replace `sleep 0.1` with `IO.select([@r])` to reliably wait for data
to arrive before asserting readability with a zero timeout. The fixed
sleep was insufficient on slow Windows CI runners, causing sporadic
failures. This matches the approach already used in test_wait_readable.

Author: k0kubun

test/io/wait/test_io_wait.rb

@@ -23,7 +23,7 @@ def test_wait
     omit 'unstable on MinGW' if /mingw/ =~ RUBY_PLATFORM
     assert_nil @r.wait(0)
     @w.syswrite "."
-    sleep 0.1
+    IO.select([@r])
     assert_equal @r, @r.wait(0)
   end