Always write unicast hosts file in tests (elastic#118121)

This commit adds the ability to restart a node within a test cluster.
The newly started node uses the same configuration and directories.
Since we use ephemeral ports, the unicast hosts list is rewritten for
all nodes once the restarted node comes back up.

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/local/DefaultLocalClusterHandle.java

@@ -176,8 +176,9 @@ public long getPid(int index) {
         return nodes.get(index).getPid();
     }
 
+    @Override
     public void stopNode(int index, boolean forcibly) {
-        nodes.get(index).stop(false);
+        nodes.get(index).stop(forcibly);
     }
 
     @Override
@@ -252,9 +253,8 @@ private void writeUnicastHostsFile() {
         execute(() -> nodes.parallelStream().forEach(node -> {
             try {
                 Path hostsFile = node.getWorkingDir().resolve("config").resolve("unicast_hosts.txt");
-                if (Files.notExists(hostsFile)) {
-                    Files.writeString(hostsFile, transportUris);
-                }
+                LOGGER.info("Writing unicast hosts file {} for node {}", hostsFile, node.getName());
+                Files.writeString(hostsFile, transportUris);
             } catch (IOException e) {
                 throw new UncheckedIOException("Failed to write unicast_hosts for: " + node, e);
             }