Drog, going back 40 years, yes 40 years, I was asked to "automate" the general ledger of a bank.
We had to have a plan, with timescales and significant points.
At these significant points, all the people whose bits of software were impacted had to sign off on the changes we had made. At that point we would run tests , just to make sure.
This continued upto the end, where everything was tested again. Every body checked it and signed off that it was ok.
There was one final check that had to be oked, and if everyone agreed, it went live.
Now, my boss was a sceptic, he said, (and I'll remember these words til I die) do a manual run for two weeks, "just to make sure". After three or four days he was convinced the new system was ok.
The point I'm trying to make is, that nowadays, all the testing of new systems is done after everything has gone live, and the public have to suffer the consequences when systems fail.