Fix bad assumption in BacktraceCleaner test [#3249 state:resolved]

Signed-off-by: NPratik Naik <pratiknaik@gmail.com>

railties/test/backtrace_cleaner_test.rb

@@ -35,7 +35,7 @@ def setup
   end
 
   test "should format installed gems correctly" do
-    @backtrace = [ "#{Gem.default_dir}/gems/nosuchgem-1.2.3/lib/foo.rb" ]
+    @backtrace = [ "#{Gem.path[0]}/gems/nosuchgem-1.2.3/lib/foo.rb" ]
     @result = @cleaner.clean(@backtrace)
     assert_equal "nosuchgem (1.2.3) lib/foo.rb", @result[0]
   end