It depends on how risk tolerant you are. If you are at a startup, the growth of your startup is often largely dependent on how quickly you can add features / onboard customers. In that context, writing tests not only slows you down from adding new features, it might make it harder to modify existing features. In addition, early customers also tend to be accepting of small bugs -- they themselves already have "taken a risk" in trusting an early startup. So testing is not really valuable -- you want to remain agile, and you won't lose much money because of a bug.
On the other hand, if you are Google, you already have found a money-printing firehose, and you /don't/ want to take on any additional unnecessary risk. Any new code needs to /not/ break existing functionality -- if it does, you might lose out of millions of revenue. In addition, your product becomes so large that it is impossible to manually test every feature. In this case, tests actually help you move /faster/ because they help you at scale automatically ensure that a change does not break anything.
While cURL does not make any money, it is solidly on the mature/Google end of the testing spectrum. It has found a footing in the open source tooling "market" and people rely on it to maintain its existing functionality. In addition, it has accumulated a fairly large surface area, so manual testing is not really feasible for every feature. So testing similarly helps cURL developers move faster (in the long run), not slower.
On the other hand, if you are Google, you already have found a money-printing firehose, and you /don't/ want to take on any additional unnecessary risk. Any new code needs to /not/ break existing functionality -- if it does, you might lose out of millions of revenue. In addition, your product becomes so large that it is impossible to manually test every feature. In this case, tests actually help you move /faster/ because they help you at scale automatically ensure that a change does not break anything.
While cURL does not make any money, it is solidly on the mature/Google end of the testing spectrum. It has found a footing in the open source tooling "market" and people rely on it to maintain its existing functionality. In addition, it has accumulated a fairly large surface area, so manual testing is not really feasible for every feature. So testing similarly helps cURL developers move faster (in the long run), not slower.