t.Log("line 1\nline 2\nline 3")

Old output:
=== RUN TestLine3
--- PASS: TestLine3 (0.00 seconds)
testing_test.go:25: 	line 1
		line 2
		line 3
		PASS

New output:
=== RUN TestLine3
--- PASS: TestLine3 (0.00 seconds)
testing_test.go:24: 	line 1
		line 2
		line 3
PASS

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6613069