Raise a helpful error in a test helper

This would've helped us debug something a little quicker

Author: maxjacobson

spec/support/helpers/analyzer_spec_helpers.rb

@@ -1,5 +1,6 @@
 module AnalyzerSpecHelpers
   def create_source_file(path, content)
+    raise "Must use in_tmpdir tag" unless @code
     File.write(File.join(@code, path), content)
   end