I admit that using real person-related data in a test database for ordinary software development (not considering fixing a production system here) might be legitimate according GDPR based on contract and consent. But the hurds are so high that I cannot think of any realistic use case that involves more than a couple of people at the maximum. Contract and consent are subject to specific requirements here. The constent must typically be voluntarily, i.e. without any pressure or coercion, it must not be coupled to another condition and it must be given for each specific purpose indivdually. And I doubt that it is permitted to completely exlude the principle of "privacy by design" in a contract. And even if this were the case, you need this consent from each customer whose data is in the database, while you must not make your service to the customer dependant on his consent.
As to legitimate interest, Recital 47 of GDPR states: "The legitimate interests of a controller, including those of a controller to which the personal data may be disclosed, or of a third party, may provide a legal basis for processing, provided that the interests or the fundamental rights and freedoms of the data subject are not overriding, taking into consideration the reasonable expectations of data subjects based on their relationship with the controller. Such legitimate interest could exist for example where there is a relevant and appropriate relationship between the data subject and the controller in situations such as where the data subject is a client or in the service of the controller. At any rate the existence of a legitimate interest would need careful assessment including whether a data subject can reasonably expect at the time and in the context of the collection of the personal data that processing for that purpose may take place. The interests and fundamental rights of the data subject could in particular override the interest of the data controller where personal data are processed in circumstances where data subjects do not reasonably expect further processing."[1]
Typically one does not have a contract about software development with a customer whose personal data is stored in a database, but about a specific service, such as for example selling something to him via a Web-shop. So if Uncle Joe is buying something from the Web-shop, can he reasonably expect that his personal data is used in developing the Web-shop software? Most likely not. Ergo, there is no legitimate interest to use his data for that pupose.
[1] https://gdpr-info.eu/recitals/no-47/
[Edit for clarity.]