testing: add file:line stamps to messages.

A single-line error looks like this:
--- FAIL: foo_test.TestFoo (0.00 seconds)
        foo_test.go:123: Foo(8) = "10" want "100"

A multi-line error looks like this:
--- FAIL: foo_test.TestFoo (0.00 seconds)
        foo_test.go:456: Foo(88) = "100"
                want "1000"

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5376057