Fix mutation function in JoinStatusTests (#92534) (#92545)

Sometimes `mutateJoinStatus()` would fail to return a different object,
e.g. if the message was very short. Also in case 4 it was mutating both
the message and the age, but should not have been mutating the message.

Author: DaveCTurner

server/src/test/java/org/elasticsearch/cluster/coordination/JoinStatusTests.java

@@ -47,16 +47,16 @@ private JoinStatus mutateJoinStatus(JoinStatus originalJoinStatus) {
                 return new JoinStatus(
                     originalJoinStatus.remoteNode(),
                     originalJoinStatus.term(),
-                    randomAlphaOfLengthBetween(0, 30),
+                    randomValueOtherThan(originalJoinStatus.message(), () -> randomAlphaOfLengthBetween(0, 30)),
                     originalJoinStatus.age()
                 );
             }
             case 3 -> {
                 return new JoinStatus(
                     originalJoinStatus.remoteNode(),
                     originalJoinStatus.term(),
-                    randomAlphaOfLength(30),
-                    randomNonNegativeTimeValue()
+                    originalJoinStatus.message(),
+                    randomValueOtherThan(originalJoinStatus.age(), this::randomNonNegativeTimeValue)
                 );
             }
             case 4 -> {