Suppress the “experimental" warnings for IO::Buffer

As this warning is emitted just once per processes, needs in each
files when parallel testing.

test/fiber/scheduler.rb

@@ -14,6 +14,14 @@ rescue LoadError
 end
 
 class Scheduler
+  experimental = Warning[:experimental]
+  begin
+    Warning[:experimental] = false
+    IO::Buffer.new(0)
+  ensure
+    Warning[:experimental] = experimental
+  end
+
   def initialize
     @readable = {}
     @writable = {}

test/ruby/test_io_buffer.rb

@@ -1,6 +1,14 @@
 # frozen_string_literal: false
 
 class TestIOBuffer < Test::Unit::TestCase
+  experimental = Warning[:experimental]
+  begin
+    Warning[:experimental] = false
+    IO::Buffer.new(0)
+  ensure
+    Warning[:experimental] = experimental
+  end
+
   def assert_negative(value)
     assert(value < 0, "Expected #{value} to be negative!")
   end