the only site i can think of that does something like this is lab tests online (http://www.labtestsonline.org/
). when ever i go to one of the pages for a lab test they often list the diseases that the test is used to diagnose. i never used it the other way around though. they do have a search box on each page, so you can input a disease and do a search and links to pages on their site come up.
there is also this page on the family practice notebook http://www.fpnotebook.com/lab.htm
which has lists of tests by medical specialty. if you look down at the "related topics from other books" area on the page you will see a series of arrows for various medical specialties. these are drop down menus. the items in these menus are links to the diagnostic tests commonly ordered by those specialties in diagnosing and treating diseases within that division of medicine. these lists are hardly exhaustive though. if you click on one of them, you are usually taken to a page that gives you information about the test. you can always try putting a labtest into the search box on their site to see if you get a hit for it.
a couple of books that i use for reference here at my computer might be helpful to you. one i just acquired. nurse's 5-minute clinical consult: diseases
just published by lippincott williams & wilkins. it has over 400 diseases listed in it. each disease is given two pages in the book where a lot of information is listed including diagnostic tests that are commonly ordered. this book also includes nursing diagnoses, outcomes, nursing interventions and patient teaching. another book with more extensive pathophysiology but only medical diseases is pathophysiology: a 2-in-1 reference for nurses
also from lippincott williams & wilkins. the clinical consult book has psych and ob/gyn conditions listed in it. the 2-in-1 reference does not.
i have other online resources for lab if you want the links, but they are not cross-referenced in the way you are wanting. if you know a patient's disease you are always going to find information about how the doctor treats it on family practice notebook and in the articles on emedicine (http://www.emedicine.com/
). emedicine articles are continuing education articles for doctors, but they often start out by presenting the incidence, pathophysiology and diagnostic tests required to diagnose the disease. you may have to register to see the articles, but it is free.