sw: use less confusing variable names in python test

Change-Id: Ie07fe4761a634c3ae55e82e1ed70c9424dc33213
sw: initialize SwDoc::mbInReading

This was causing intermittent failure at least in sw_python test.
(regression from 5494954b269267f6ee3bdd5ac73e7513fa69978f)

Change-Id: I8452536cfbb7bc5bd07e6405bc255e31a5007185
sw: assert that UnlockExpFlds() isn't called when it's not locked

Change-Id: Ibe2394978b776dd4a91a253856040fc45f81c7e9