Set exit code variable without calling process.exit

`process.exit` quits immediately, without allowing buffered output from
say, a `console.log` to be printed. See these Node.js issues:

nodejs/node#3669
nodejs/node#3170
nodejs/node#2972 (comment)

bin/fuzzy-tester

@@ -29,7 +29,7 @@ var analyze_results = require( '../lib/analyze_results' );
     // display results using the correct output generator
     var return_code = config.outputGenerator(evaled_results, config, testSuites);
 
-    process.exit(return_code);
+    process.exitCode = return_code;
   };
 
   // find all test suites and test cases that will be run