test/test_pair: replace sleep with IO.select

The sleep was to ensure that the SSLSocket#read_nonblock will get
close_notify alert. A simple IO.select will suffice.

Author: rhenium

test/test_pair.rb

@@ -218,7 +218,7 @@ def test_read_nonblock
       assert_nothing_raised("[ruby-core:20298]") { ret = s2.read_nonblock(10) }
       assert_equal("def\n", ret)
       s1.close
-      sleep 0.1
+      IO.select([s2])
       assert_raise(EOFError) { s2.read_nonblock(10) }
     }
   end
@@ -234,7 +234,7 @@ def test_read_nonblock_no_exception
       assert_nothing_raised("[ruby-core:20298]") { ret = s2.read_nonblock(10, exception: false) }
       assert_equal("def\n", ret)
       s1.close
-      sleep 0.1
+      IO.select([s2])
       assert_equal(nil, s2.read_nonblock(10, exception: false))
     }
   end