Fix Kernel#global_variables for $1-$9

test/t/kernel.rb

@@ -479,7 +479,7 @@ assert('Kernel#!=') do
   assert_false (str2 != str1)
 end
 
-# operator "!~" is defined in ISO Ruby 11.4.4. 
+# operator "!~" is defined in ISO Ruby 11.4.4.
 assert('Kernel#!~') do
   x = "x"
   def x.=~(other)
@@ -511,6 +511,13 @@ assert('Kernel#respond_to_missing?') do
   assert_false Test4RespondToMissing.new.respond_to?(:no_method)
 end
 
+assert('Kernel#global_variables') do
+  variables = global_variables
+  1.upto(9) do |i|
+    assert_equal variables.include?(:"$#{i}"), true
+  end
+end
+
 assert('stack extend') do
   def recurse(count, stop)
     return count if count > stop