What steps will reproduce the problem?

This is a feature request to expose the call depth that testing will log failures at.
Consider this code

http://play.golang.org/p/MhVJrCk2oh

What is the expected output? What do you see instead?

What we get


=== RUN TestSomething
--- FAIL: TestSomething (0.00 seconds)
        tt_test.go:8: an error
FAIL
exit status 1
FAIL    tt      0.002s

What i'd like


=== RUN TestSomething
--- FAIL: TestSomething (0.00 seconds)
        tt_test.go:16: an error
FAIL
exit status 1
FAIL    tt      0.002s

Can a method be added to testing.common to allow the the caller of Fatal/Error/Log{,f}
to specify the number of stackframes to unwind.

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