add a test that exposes exc_info being set in one greenlet is visible in another

greentest/test__switch_exc_info.py

+import gevent
+import sys
+
+
+def hello():
+    assert sys.exc_info() == (None, None, None), sys.exc_info()
+
+error = Exception('hello')
+
+try:
+    raise error
+except:
+    gevent.spawn(hello).join()
+    try:
+        raise
+    except Exception, ex:
+        assert ex is error, (ex, error)