Unless you checked every possible result for every possible function call you use then the 3rd party library could have been changed in such a way which would b

Unless you checked every possible result for every possible function call you use then the 3rd party library could have been changed in such a way which would break your program. There's no way to prevent such situation in general, e.g when the input value space is non-finite. Other kinds of specification would seem very hard to test, for example rendering a 3D scene, if we used a bitmap as test data then we'd get false positives when the rendering is improved in some minor way, and the spec will fail, we'd need to use some perceptual metric for image similarity instead. Version numbers aren't so bad, given their ease of use.