Use a new test loader for each queue item.

The loader's ContextSuiteFactory keeps a reference to all of the test
suites, even after they've been run, which consumes more and more
memory over time.

This change creates a new loader for each suite that's dispatched to
the worker, which allows the GC to free the old test suites.

nose/plugins/multiprocess.py

@@ -668,8 +668,6 @@ def __runner(ix, testQueue, resultQueue, currentaddr, currentstart,
     config.plugins.configure(config.options,config)
     config.plugins.begin()
     log.debug("Worker %s executing, pid=%d", ix,os.getpid())
-    loader = loaderClass(config=config)
-    loader.suiteClass.suiteClass = NoSharedFixtureContextSuite
 
     def get():
         return testQueue.get(timeout=config.multiprocess_timeout)
@@ -702,6 +700,8 @@ def batch(result):
             log.exception('Worker %d STOPPED',ix)
             break
         result = makeResult()
+        loader = loaderClass(config=config)
+        loader.suiteClass.suiteClass = NoSharedFixtureContextSuite
         test = loader.loadTestsFromNames([test_addr])
         test.testQueue = testQueue
         test.tasks = []