[Tests] Change cluster scope in CorruptedFileIT and FlushIT (#37229)

These tests failed on CI multiple times in the past weeks because they use a 
test cluster with a SUITE scope that recreates nodes between tests. With such 
a scope, nodes can be recreated in between test executions and can inherit a 
node id from a previous test execution, while they are assigned a random data 
path. With the successive node recreations it is possible that a newly recreated 
node shares the same node id (but different data path) as a non recreated node.

This commit changes the cluster scope of the CorruptedFileIT and FlushIT 
tests which often fail.

The failure is reproducable with :

./gradlew :server:integTest -Dtests.seed=EF3A50C225CF377 
-Dtests.class=org.elasticsearch.index.store.CorruptedFileIT 
-Dtests.security.manager=true -Dtests.locale=th-TH-u-nu-thai-x-lvariant-TH -Dtests.timezone=America/Rio_Branco 
-Dcompiler.java=11 -Druntime.java=8

server/src/test/java/org/elasticsearch/index/store/CorruptedFileIT.java

@@ -107,7 +107,7 @@ import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.notNullValue;
 
-@ESIntegTestCase.ClusterScope(scope = ESIntegTestCase.Scope.SUITE)
+@ESIntegTestCase.ClusterScope(scope = ESIntegTestCase.Scope.TEST)
 public class CorruptedFileIT extends ESIntegTestCase {
 
     @Override

server/src/test/java/org/elasticsearch/indices/flush/FlushIT.java

@@ -64,6 +64,7 @@ import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.nullValue;
 
+@ESIntegTestCase.ClusterScope(scope = ESIntegTestCase.Scope.TEST)
 public class FlushIT extends ESIntegTestCase {
 
     public void testWaitIfOngoing() throws InterruptedException {