Suppress Ruby warnings in certain backtrace filtering tests

Since they're sensitive to the warnings, and the warnings are
not relevant to the tests, we can suppress them to keep the tests
simple.

test/irb/test_irb.rb

@@ -825,6 +825,13 @@ def build_irb
   end
 
   class BacktraceFilteringTest < TestIRB::IntegrationTestCase
+    def setup
+      super
+      # These tests are sensitive to warnings, so we disable them
+      original_rubyopt = ENV["RUBYOPT"] || ""
+      @envs["RUBYOPT"] = original_rubyopt + " -W0"
+    end
+
     def test_backtrace_filtering
       write_ruby <<~'RUBY'
         def foo