This sounds like a needlessly strict interpretation of GDPR. Taken from the UK regulator's site:
> The lawful bases for processing are set out in Article 6 of the UK GDPR. At least one of these must apply whenever you process personal data:
> (a) Consent: the individual has given clear consent for you to process their personal data for a specific purpose.
> (b) Contract: the processing is necessary for a contract you have with the individual, or because they have asked you to take specific steps before entering into a contract.
> (f) Legitimate interests: the processing is necessary for your legitimate interests or the legitimate interests of a third party, unless there is a good reason to protect the individual’s personal data which overrides those legitimate interests. (This cannot apply if you are a public authority processing data to perform your official tasks.)
...
> Legitimate interests is the most flexible lawful basis for processing, but you cannot assume it will always be the most appropriate. It is likely to be most appropriate where you use people’s data in ways they would reasonably expect and which have a minimal privacy impact, or where there is a compelling justification for the processing.
> The processing must be necessary. If you can reasonably achieve the same result in another less intrusive way, legitimate interests will not apply.
> You must include details of your legitimate interests in your privacy information.
I included the legitimate interests bits because they seem most relevant to testing, but even if testing is not considered "necessary" in a particular use case, there still remain at least two more criteria that might satisfy the use of live data in testing, including explicit user consent. Much of the focus of GDPR is on privacy-invasive intrusive processing and prevention of harm, I think a lot of fuss around it can be dispelled when viewed from this angle.