2 snapshot threads for large blob test (#91226)

In 8.5, the default snapshot thread pool has 1 thread when the
"Timeout with large blobs" runs with the
-Dtests.configure_test_clusters_with_one_processor=true
pipeline. This makes the 1sec timeout response to never be sent
out (because the 1 thread is busy with the slow blob), while with
2 threads it can be sent out.

Fixes #90353

x-pack/plugin/snapshot-repo-test-kit/qa/rest/build.gradle

@@ -16,6 +16,7 @@ testClusters.matching { it.name == "yamlRestTest" }.configureEach {
   testDistribution = 'DEFAULT'
   setting 'path.repo', repoDir.absolutePath
   setting 'xpack.security.enabled', 'false'
+  setting 'thread_pool.snapshot.max', '2'
 }
 
 tasks.named('yamlRestTestTestingConventions').configure {