runtime/trace: better output when test fails

Change-Id: I108d15eb4cd25904bb76de4ed7548c039c69d1a3
Reviewed-on: https://go-review.googlesource.com/40590
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/trace/trace_stack_test.go

@@ -276,9 +276,10 @@ func TestTraceSymbolize(t *testing.T) {
 				continue
 			}
 			for _, f := range ev.Stk {
-				t.Logf("  %v:%v", f.Fn, f.Line)
+				t.Logf("  %v :: %s:%v", f.Fn, f.File, f.Line)
 			}
 			t.Logf("---")
 		}
+		t.Logf("======")
 	}
 }