time: Fix missing colon when formatting time zone offsets with seconds

When formatting time zone offsets with seconds using the stdISO8601Colon
and stdNumColon layouts, the colon was missing between the hour and minute
parts.

Fixes #8497.

LGTM=r
R=golang-codereviews, iant, gobot, r
CC=golang-codereviews
https://golang.org/cl/126840043
diff --git a/src/pkg/time/format_test.go b/src/pkg/time/format_test.go
index 3bc8f42..a7c6d55 100644
--- a/src/pkg/time/format_test.go
+++ b/src/pkg/time/format_test.go
@@ -502,10 +502,11 @@
 }
 
 func TestFormatSecondsInTimeZone(t *testing.T) {
-	d := Date(1871, 9, 17, 20, 4, 26, 0, FixedZone("LMT", -(34*60+8)))
-	timestr := d.Format("2006-01-02T15:04:05Z070000")
-	expected := "1871-09-17T20:04:26-003408"
-	if timestr != expected {
-		t.Errorf("Got %s, want %s", timestr, expected)
+	for _, test := range secondsTimeZoneOffsetTests {
+		d := Date(1871, 1, 1, 5, 33, 2, 0, FixedZone("LMT", test.expectedoffset))
+		timestr := d.Format(test.format)
+		if timestr != test.value {
+			t.Errorf("Format = %s, want %s", timestr, test.value)
+		}
 	}
 }