packetbeat/route: fix test failure on Windows 2022 (#39822)

Windows 2022 appears to have added new text to the output of netsh. The text
is interpreted as having three fields. This impact on the behaviour of the
field splitting which expects five.

- Originally this would cause a panic due to bounds checking. This was fixed
  in #39757 by erroring on an unexpectedly short line.
- The fieldsN helper expects lines to be n fields or longer, but does not
  terminate on lines with fewer fields.

So treat the first blank line found in the table as the end of the table and
fix the fieldsN helper.

(cherry picked from commit 35f8d09)

CHANGELOG-developer.next.asciidoc

@@ -97,6 +97,7 @@ The list below covers the major changes between 7.0.0-rc2 and main only.
 - Fix copy arguments for strict aligned architectures. {pull}36976[36976]
 - Fix panic when more than 32767 pipeline clients are active. {issue}38197[38197] {pull}38556[38556]
 - Skip flakey metrics test on windows in filebeat httpjson input. {issue}39676[39676] {pull}39678[39678]
+- Fix flakey test on Windows 2022 in packetbeat/route. {issue}39698[39698] {pull}39822[39822]
 
 ==== Added
 

packetbeat/route/route_test.go

@@ -32,7 +32,6 @@ import (
 )
 
 func TestDefault(t *testing.T) {
-	t.Skip("Flaky test: https://github.com/elastic/beats/issues/39698")
 	for _, family := range []int{syscall.AF_INET, syscall.AF_INET6} {
 		wantIface, wantIndex, wantErr := defaultRoute(family)
 		if wantErr != nil && wantErr != ErrNotFound {

packetbeat/route/route_windows_test.go

@@ -54,6 +54,9 @@ func defaultRoute(af int) (name string, index int, err error) {
 	for inTable := false; sc.Scan(); {
 		f := strings.Fields(sc.Text())
 		if len(f) == 0 {
+			if inTable {
+				break
+			}
 			continue
 		}
 		if !inTable {
@@ -91,6 +94,9 @@ func defaultRoute(af int) (name string, index int, err error) {
 	for inTable := false; sc.Scan(); {
 		f := fieldsN(sc.Text(), 5)
 		if len(f) == 0 {
+			if inTable {
+				break
+			}
 			continue
 		}
 		if !inTable {
@@ -149,6 +155,7 @@ func fieldsN(s string, n int) []string {
 	}
 	var f []string
 	for s != "" {
+		l := len(s)
 		for i, r := range s {
 			if unicode.IsSpace(r) {
 				f = append(f, s[:i])
@@ -162,7 +169,7 @@ func fieldsN(s string, n int) []string {
 				break
 			}
 		}
-		if len(f) == n-1 {
+		if len(f) == n-1 || len(s) == l {
 			break
 		}
 	}