Introduce test.test_support.TransientResource. It's a context manager to

surround calls to resources that may or may not be available.  Specifying the
expected exception and attributes to be raised if the resource is not available
prevents overly broad catching of exceptions.

This is meant to help suppress spurious failures by raising
test.test_support.ResourceDenied if the exception matches.  It would probably
be good to go through the various network tests and surround the calls to catch
connection timeouts (as done with test_socket_ssl in this commit).

Lib/test/test_support.py

@@ -312,6 +312,31 @@ class EnvironmentVarGuard(object):
         for unset in self._unset:
             del self._environ[unset]
 
+class TransientResource(object):
+
+    """Raise ResourceDenied if an exception is raised while the context manager
+    is in effect that matches the specified exception and attributes."""
+
+    def __init__(self, exc, **kwargs):
+        self.exc = exc
+        self.attrs = kwargs
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, type_=None, value=None, traceback=None):
+        """If type_ is a subclass of self.exc and value has attributes matching
+        self.attrs, raise ResourceDenied.  Otherwise let the exception
+        propagate (if any)."""
+        if type_ is not None and issubclass(self.exc, type_):
+            for attr, attr_value in self.attrs.iteritems():
+                if not hasattr(value, attr):
+                    break
+                if getattr(value, attr) != attr_value:
+                    break
+            else:
+                raise ResourceDenied("an optional resource is not available")
+
 
 #=======================================================================
 # Decorator for running a function in a different locale, correctly resetting