strings: add test for Count

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12541050

src/pkg/strings/strings_test.go

@@ -1010,6 +1010,30 @@ func TestEqualFold(t *testing.T) {
 	}
 }
 
+var CountTests = []struct {
+	s, sep string
+	num    int
+}{
+	{"", "", 1},
+	{"", "notempty", 0},
+	{"notempty", "", 9},
+	{"smaller", "not smaller", 0},
+	{"12345678987654321", "6", 2},
+	{"611161116", "6", 3},
+	{"notequal", "NotEqual", 0},
+	{"equal", "equal", 1},
+	{"abc1231231123q", "123", 3},
+	{"11111", "11", 2},
+}
+
+func TestCount(t *testing.T) {
+	for _, tt := range CountTests {
+		if num := Count(tt.s, tt.sep); num != tt.num {
+			t.Errorf("Count(\"%s\", \"%s\") = %d, want %d", tt.s, tt.sep, num, tt.num)
+		}
+	}
+}
+
 func makeBenchInputHard() string {
 	tokens := [...]string{
 		"<a>", "<p>", "<b>", "<strong>",