thanks for pointing me at this issue! The unit tests stopped working in Xcode 4.3. Through trial and error, I found out what the problem was and fixed it.
If anyone is coming here through Google and is having the same problem, here's what you have to do to fix the problem:
The key is the Build Setting "Framework Search Paths". It needs to contain the following values, in this order :
Furthermore, I linked the project to "SenTestingKit.framework" under "Build Phases - Link Binary with Libraries". Before, the framework was linked through arguments in "Other Linker Flags".