test_live_restore.cpp was printing the error trace

__session_truncate, 1632: the truncate method should be passed either a URI or start/stop cursors, but not both: Invalid argument 

but not failing tests. This was because we missed adding a testutil_check around the function call and never checked the return value.

This ticket adds the proper testutil_check wrapper, fixes the truncate call, and also correctly positions the random cursor which was declared but never positioned.