testing: use strings.SplitSeq and bytes.SplitSeq

To simplify the code. This is a follow up for the CL 646216.

src/testing/slogtest/example_test.go

@@ -23,7 +23,7 @@ func Example_parsing() {
 
 	results := func() []map[string]any {
 		var ms []map[string]any
-		for _, line := range bytes.Split(buf.Bytes(), []byte{'\n'}) {
+		for line := range bytes.SplitSeq(buf.Bytes(), []byte{'\n'}) {
 			if len(line) == 0 {
 				continue
 			}

src/testing/testing.go

@@ -2501,7 +2501,7 @@ func (m *M) stopAlarm() {
 }
 
 func parseCpuList() {
-	for _, val := range strings.Split(*cpuListStr, ",") {
+	for val := range strings.SplitSeq(*cpuListStr, ",") {
 		val = strings.TrimSpace(val)
 		if val == "" {
 			continue

src/testing/testing_test.go

@@ -894,7 +894,7 @@ func TestRunningTestsInCleanup(t *testing.T) {
 
 func parseRunningTests(out []byte) (runningTests []string, ok bool) {
 	inRunningTests := false
-	for _, line := range strings.Split(string(out), "\n") {
+	for line := range strings.SplitSeq(string(out), "\n") {
 		if inRunningTests {
 			// Package testing adds one tab, the panic printer adds another.
 			if trimmed, ok := strings.CutPrefix(line, "\t\t"); ok {