test_file_exhaustive.rb: remove useless assignment

* test/ruby/test_file_exhaustive.rb (make_tmp_filename): not
  assign to instance variable, @hardlinkfile.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49661 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_file_exhaustive.rb

@@ -46,7 +46,7 @@ class TestFileExhaustive < Test::Unit::TestCase
   end
 
   def make_tmp_filename(prefix)
-    @hardlinkfile = @dir + "/" + prefix + File.basename(__FILE__) + ".#{$$}.test"
+    "#{@dir}/#{prefix}#{File.basename(__FILE__)}.#{$$}.test"
   end
 
   def test_path