Add tests and fix bugs in the new cross-thread lock resolution code.

When an unlock fails, do not leak file descriptors (ticket #1611).
But we really ought to report SQLITE_MISUSE or some other error instead
of just returning SQLITE_OK. (CVS 2945)

FossilOrigin-Name: f68e05cb2be65fad43fac823b2a9c53b6d2e797d