Suppress the “experimental" warnings for `IO::Buffer`

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

1 files changed, 8 insertions, 0 deletions

diff --git a/test/ruby/test_io_buffer.rb b/test/ruby/test_io_buffer.rb
index 57d1867b25..f7c175b589 100644
--- a/test/ruby/test_io_buffer.rb
+++ b/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