[Test] Single-project-only resolver should not support multi-project

In MP-1749, we differentiate between single-project and single-project
only resolvers. The later should not support multi-project.

Author: ywangd

test/framework/src/main/java/org/elasticsearch/cluster/project/TestProjectResolvers.java

@@ -144,7 +144,7 @@ public <E extends Exception> void executeOnProject(ProjectId otherProjectId, Che
 
             @Override
             public boolean supportsMultipleProjects() {
-                return true;
+                return only == false;
             }
         };
     }