irb/test_init.rb: add test to ensure $0

is not changed.

At first `ARGV.unshift('something')` was suggested for r61149,
but it wasn't sufficient because it modifies $0.

Not only to preserve ARGV, but also r61149 intends to preserve $0.
This test prevents future breakage of the behavior.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61167 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/irb/test_init.rb

@@ -12,6 +12,12 @@ module TestIRB
       end
     end
 
+    def test_setup_with_empty_argv_does_not_change_dollar0
+      orig = $0.dup
+      IRB.setup(eval("__FILE__"), argv: [])
+      assert_equal orig, $0
+    end
+
     private
 
     def with_argv(argv)