Add testcase for internal used Comparator in ClientCookieEncoder (#9897)

Motivation:

https://github.com/netty/netty/pull/9883 added a bug-fix for the Comparator in ClientCookieEncoder but did not add a testcase.

Modifications:

- Add testcase
- Simplify code

Result:

Include a test to ensure we not regress.

codec-http/src/main/java/io/netty/handler/codec/http/cookie/ClientCookieEncoder.java

@@ -92,7 +92,8 @@ public final class ClientCookieEncoder extends CookieEncoder {
      * Sort cookies into decreasing order of path length, breaking ties by sorting into increasing chronological
      * order of creation time, as recommended by RFC 6265.
      */
-    private static final Comparator<Cookie> COOKIE_COMPARATOR = (c1, c2) -> {
+    // package-private for testing only
+    static final Comparator<Cookie> COOKIE_COMPARATOR = (c1, c2) -> {
         String path1 = c1.path();
         String path2 = c2.path();
         // Cookies with unspecified path default to the path of the request. We don't
@@ -102,13 +103,10 @@ public final class ClientCookieEncoder extends CookieEncoder {
         // limited use.
         int len1 = path1 == null ? Integer.MAX_VALUE : path1.length();
         int len2 = path2 == null ? Integer.MAX_VALUE : path2.length();
-        int diff = len2 - len1;
-        if (diff != 0) {
-            return diff;
-        }
+
         // Rely on Arrays.sort's stability to retain creation order in cases where
         // cookies have same path length.
-        return 0;
+        return len2 - len1;
     };
 
     /**

codec-http/src/test/java/io/netty/handler/codec/http/cookie/ClientCookieEncoderTest.java

@@ -51,4 +51,16 @@ public class ClientCookieEncoderTest {
     public void testRejectCookieValueWithSemicolon() {
         ClientCookieEncoder.STRICT.encode(new DefaultCookie("myCookie", "foo;bar"));
     }
+
+    @Test
+    public void testComparatorForSamePathLength() {
+        Cookie cookie = new DefaultCookie("test", "value");
+        cookie.setPath("1");
+
+        Cookie cookie2 = new DefaultCookie("test", "value");
+        cookie2.setPath("2");
+
+        assertEquals(0, ClientCookieEncoder.COOKIE_COMPARATOR.compare(cookie, cookie2));
+        assertEquals(0, ClientCookieEncoder.COOKIE_COMPARATOR.compare(cookie2, cookie));
+    }
 }