Fix test failures when Python is compiled without thread support.

Lib/trace.py

@@ -502,15 +502,7 @@ def __init__(self, count=1, trace=1, countfuncs=0, countcallers=0,
     def run(self, cmd):
         import __main__
         dict = __main__.__dict__
-        if not self.donothing:
-            threading.settrace(self.globaltrace)
-            sys.settrace(self.globaltrace)
-        try:
-            exec cmd in dict, dict
-        finally:
-            if not self.donothing:
-                sys.settrace(None)
-                threading.settrace(None)
+        self.runctx(cmd, dict, dict)
 
     def runctx(self, cmd, globals=None, locals=None):
         if globals is None: globals = {}