In the Test navigator (command-5), right click on any test and select "Profile [test name]". This will bring up Instruments and allow you to profile the specific test.
after following your instructions, when I start the time profiler and I try to record I have this error:
The operation couldn’t be completed. Application "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Xcode/Agents/xctest" is unknown to FrontBoard. : Failed to launch process with bundle identifier '/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Xcode/Agents/xctest'
The Profile scheme has already Debug as Build Configuration.