[qa] pull-tester: Fix assertion and check for run_parallel

qa/pull-tester/rpc-tests.py

@@ -231,7 +231,7 @@ def runtests():
     if coverage:
         flags.append(coverage.flag)
 
-    if len(test_list) > 1:
+    if len(test_list) > 1 and run_parallel > 1:
         # Populate cache
         subprocess.check_output([RPC_TESTS_DIR + 'create_cache.py'] + flags)
 
@@ -291,7 +291,7 @@ class RPCTestHandler:
                                                stdout=subprocess.PIPE,
                                                stderr=subprocess.PIPE)))
         if not self.jobs:
-            raise IndexError('%s from empty list' % __name__)
+            raise IndexError('pop from empty list')
         while True:
             # Return first proc that finishes
             time.sleep(.5)