Fix threadpool setting test for system_write (#63706)

This commit fixes the UpdateThreadPoolSettingsTests to be aware of the
hard limit on the maximum size of the system_write executor. This
executor has a hard limit that matches the write executor, which is
the number of allocated processors.

Closes #63131
Backport #63700

server/src/test/java/org/elasticsearch/threadpool/UpdateThreadPoolSettingsTests.java

@@ -87,7 +87,7 @@ public void testWriteThreadPoolsMaxSize() throws InterruptedException {
     }
 
     private static int getExpectedThreadPoolSize(Settings settings, String name, int size) {
-        if (name.equals(ThreadPool.Names.WRITE)) {
+        if (name.equals(ThreadPool.Names.WRITE) || name.equals(Names.SYSTEM_WRITE)) {
             return Math.min(size, EsExecutors.allocatedProcessors(settings));
         } else {
             return size;