Several lens test sites have sprouted in recent years. The test results published by these sites are taken seriously by many users, therefore, it is important to understand what value we can attach to these tests.
Firstly, the lenses are tested at a particular focal plane, i.e., the distance between the lens and the test object is fixed. As we all know, in the real world, a lens is focused anywhere between its closest focusing distance and infinity. To test a lens at a single focal plane and then to extrapolate from this an overall judgment of the lens performance is flawed in my opinion.
Secondly, certain lenses are designed for specific types of photography. Such as macro lenses for macro photography, short telephoto lenses for portrait photography, long telephoto for sports/wildlife etc. To test a lens outside of its usage context seems pretty pointless. One has to understand what purpose the lens is to serve, and devise tests accordingly.
I suggest that anyone interested in lens design should read the articles published under Nikkor Thousand and One Nights. Each article describes the goals that were aimed for during the design of a particular lens. This gives you an appreciation of why one-size-fits-all type of testing is so inadequate.
In my experience the real life performance of a lens cannot be judged by any of the tests published online. It is best to try out a lens yourself, perhaps rent it from a rental shop before buying it.