Good idea but
1) I've created a couple of Flows and I dont know hot to add a recurrence step as first step without having to recreate those flows
2) Which are the recurrence values for "run now, only once"?
1) You have to create a new Flow. I think you can change everything except the Trigger.
2) I think that those values dont exist. What I've been doing is setting 1 minute frequency let the flow run and then stop it (Turning it Off) in the My flows list: https://flow.microsoft.com/manage/flows.
Flow run depends on the trigger used in the flow. If you are specifically interested testing a flow then this feature is already there. When you save a flow you would see a screen with a message which will ask you to perform the trigger action and will wait for you to perform the action.
For example in case of file triggers you need to add a new file to the source.
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