I have a journey built and I'm wondering the easiest way to test it.
Would you create a replicated test Data Extension and copy the journey?
There are many ways to tackle this but Salesforce has NOT implemented a feature that allows you to automate the testing of journeys or entry data with minimal effort required.
You can duplicate your entry source with test records and apply the test entry source for testing in the mean time, that way no live records will enter the journey.
You can keep the live entry source but alter the SQL Query Activity that populates the entry source to only allow test records to enter the journey.
You can add a filter or a decision split at the start of your journey that only allows test records to enter based on a test value you provided and that will allow all live records to instantly exit the journey and not be affected .
NOTE: Be mindful about this approach because you may now have a watermark for your records depending on how you have setup your journey.
I never really find the need to Copy a journey, I only focus on making sure live data does not flow through my testing process.
Be mindful on wait periods in your journey, for testing purposes you might want to alter the days to minutes, or hours to minutes depending on how much time you have to test your journey.
As you can see you have a few options, there may be more ways to do this but this is how I would test my journeys using test data, just remember I would keep any 1 of the 3 that would best fit the setup of the journey you will be testing.