Introduced new integration test helper method for safely updating model
file contents in a synchronized manner and reworked all affected
integration tests to let them use that helper method; made sure that all
integration test-initiated model-affecting operations are invoked
asynchronously to eliminate risk of deadlocks; made sure that model
files being changed in tests don't end up being out of sync by forcing a
local refresh in newly introduced test helper method for updating model
file contents