testing: fix Helper line number change

parent's helperNames has not been set when frameSkip called, moving
helperNames initilazing to frameSkip.

Fixes #44887

Author: qingyunha

src/testing/helper_test.go

@@ -49,6 +49,44 @@ helperfuncs_test.go:60: 10
 	}
 }
 
+func TestTBHelperLineNumer(t *T) {
+	var buf bytes.Buffer
+	ctx := newTestContext(1, newMatcher(regexp.MatchString, "", ""))
+	t1 := &T{
+		common: common{
+			signal: make(chan bool),
+			w:      &buf,
+		},
+		context: ctx,
+	}
+	t1.Run("Test", func(t *T) {
+		helperA := func(t *T) {
+			t.Helper()
+			t.Run("subtest", func(t *T) {
+				t.Helper()
+				t.Fatal("fatal error message")
+			})
+		}
+		helperA(t)
+	})
+
+	want := `--- FAIL: Test (?s)
+--- FAIL: Test/subtest (?s)
+helper_test.go:70: fatal error message
+`
+	lines := strings.Split(buf.String(), "\n")
+	durationRE := regexp.MustCompile(`\(.*\)$`)
+	for i, line := range lines {
+		line = strings.TrimSpace(line)
+		line = durationRE.ReplaceAllString(line, "(?s)")
+		lines[i] = line
+	}
+	got := strings.Join(lines, "\n")
+	if got != want {
+		t.Errorf("got output:\n\n%v\nwant:\n\n%v", got, want)
+	}
+}
+
 func TestTBHelperParallel(t *T) {
 	var buf bytes.Buffer
 	ctx := newTestContext(1, newMatcher(regexp.MatchString, "", ""))