Reduce the number of times IO is passed in send_io/recv_io test

Since Linux 4.5, sendmsg(2) fails with ETOOMANYREFS if the number of
"in-flight" IOs, which has been sent by sendmsg(2) but has not yet
accepted by recvmsg(2), exceeds the RLIMIT_NOFILE resource limit.

https://rubyci.s3.amazonaws.com/arm64-neoverse-n1/ruby-master/log/20231025T090004Z.fail.html.gz
```
  1) Error:
TestSocket_UNIXSocket#test_fd_passing_race_condition:
Errno::ETOOMANYREFS: Too many references: cannot splice - sendmsg(2)
```

This change reduces the number of times of IO passing under 1024,
which is a default limit in many environments.

test/socket/test_unix.rb

@@ -152,7 +152,7 @@ class TestSocket_UNIXSocket < Test::Unit::TestCase
     lock = Thread::Mutex.new
     nr = 0
     x = 2
-    y = 1000
+    y = 400
     begin
       s1.send_io(nil)
     rescue NotImplementedError
@@ -167,12 +167,7 @@ class TestSocket_UNIXSocket < Test::Unit::TestCase
           true
         end
       end
-      begin
-        (x * y).times { s1.send_io r1 }
-      rescue Errno::ETOOMANYREFS => e
-        # for arm64-neoverse-n1
-        omit e.message
-      end
+      (x * y).times { s1.send_io r1 }
       assert_equal([true]*x, thrs.map { |t| t.value })
       assert_equal x * y, nr
     ensure