testing: fix extra tabs when t.Log("string")

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

src/pkg/testing/testing.go

@@ -160,6 +160,9 @@ func decorate(s string) string {
 	fmt.Fprintf(buf, "%s:%d: ", file, line)
 
 	lines := strings.Split(s, "\n")
+	if l := len(lines); l > 1 && lines[l-1] == "" {
+		lines = lines[:l-1]
+	}
 	for i, line := range lines {
 		if i > 0 {
 			buf.WriteByte('\n')
@@ -172,10 +175,7 @@ func decorate(s string) string {
 		}
 		buf.WriteString(line)
 	}
-	if l := len(s); l > 0 && s[len(s)-1] != '\n' {
-		// Add final new line if needed.
-		buf.WriteByte('\n')
-	}
+	buf.WriteByte('\n')
 	return buf.String()
 }