Suppress "Switch to inspect mode" messages

This message is displayed if STDIN is not a tty.  The parallel test is
the case.

Author: nobu

test/irb/test_completion.rb

@@ -271,6 +271,7 @@ def setup
       save_encodings
       @verbose, $VERBOSE = $VERBOSE, nil
       IRB.init_config(nil)
+      IRB.conf[:VERBOSE] = false
       IRB.conf[:MAIN_CONTEXT] = IRB::Context.new(IRB::WorkSpace.new(binding))
     end