(libT) #5276 'regression in nightly: assert (tr_sessionIsLocked (session))': add unit test (currently failing) to reproduce this error

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13979 f4695dd4-2c0a-0410-b89c-da849a56a58e
(libT) #5276 'regression in nightly: assert (tr_sessionIsLocked (session))': fix the bug; tests now pass again

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13980 f4695dd4-2c0a-0410-b89c-da849a56a58e