(#1384) Test db.sync cancel function

Add a simple test to see of db.sync cancel function is working.

Not only does db.sync cancel function appear not to be working at the moment. The test of db.sync interferes with subsequent tests somehow. Possibly by causing failure when the subsequent test attempts to initialize the test database, including destroying that left by the previous test, but I'm not at all certain about this. What I do know is that the db.sync cancel function sometimes is called and failed after a subsequent test starts.