I just read a very interesting research paper entitled, "The Impact of Test Suite Granularity on the Cost-Effectiveness of Regression Testing" by Gregg Rothermel et al.  In it the authors examine the impact of test suite granularity on several metrics.  The two most interesting are the impacts on running time and the impact on bug finding.  In both cases, they found that larger test suites were better than small ones.

by Miško Hevery

We talked about how it is important to separate the new operators from the application logic. This separation forces your code to have factories which are responsible for wiring your application together. By separating these responsibilities the tests can always wire together a subset of an application with key components replaced for friendlies making testing easier and more focused.