Stops file descriptor leaking in tests

During a test run serveral hundred WSGI servers are started, used for a
single testcase and then stopped. We were not properly killing the
greenthread in cases where we started the server, but the thread had not
started.

The problem is that a greenthead evaluates True or False depending if it
has already started running. We really just need a check to make sure
start() was called on the server. In this patch if server.greenthread is
not None that means that the server was started.

There are still other places where descriptors are leaking.

Change-Id: I06cddb97553cb2d25b7d0447634f05e2f6b163fe
Merge "Stops file descriptor leaking in tests"