I looked into this. There doesn't seem to be an easy fix from inside the "testing" package. A look inside the call stack reveals -

testing.(*common).decorate /home/agniva/play/gosource/go/src/testing/testing.go 362
testing.(*common).log /home/agniva/play/gosource/go/src/testing/testing.go 559
testing.(*common).Log /home/agniva/play/gosource/go/src/testing/testing.go 566
testing.(*common).Log-fm /home/agniva/play/gosource/go/src/encoding/hex/hex_test.go 29
encoding/hex.TestLineNumber /home/agniva/play/gosource/go/src/encoding/hex/hex_test.go 44
testing.tRunner /home/agniva/play/gosource/go/src/testing/testing.go 755
runtime.goexit /home/agniva/play/gosource/go/src/runtime/asm_amd64.s 2320

(I have put the test code inside hex_test.go. Let's ignore that)

The testing.(*common).Log-fm indicates that a closure is being created the moment something like fn := t.Log is done.

And what is more interesting is it persists across function calls. Notice that the Log-fm line is at 29 and the actual call is at 44. That is because it is happening in another Test function altogether.

As a stripped down version, I have something like

func TestEncode(t *testing.T) {
	_ = t.Log  -> L29
        ...
        ...
}


func TestLineNumber(t *testing.T) {
	log := t.Log
	log("test") -> L44
}

So, even if I execute only the TestLineNumber function, the stack shows that the closure is being created at L29.

As of 1.9 this should now be possible, at the cost of being slightly more awkward, by writing

    log := func(args ...interface{}) {
        t.Helper()
        t.Log(args...)
    }

Closing because this is kind of special purpose and there is (now) a workaround.