InternalTestCluster shouldn't auto heal an active disruption when a new one is set

Instead people should explicitly clear the existing one so it's clear what's going on.

core/src/test/java/org/elasticsearch/discovery/DiscoveryWithServiceDisruptionsIT.java

@@ -1213,7 +1213,7 @@ public class DiscoveryWithServiceDisruptionsIT extends ESIntegTestCase {
         for (String node : nodes) {
             assertNoMaster(node);
         }
-        isolateAllNodes.stopDisrupting();
+        internalCluster().clearDisruptionScheme();
         ensureStableCluster(3);
         final String preferredMasterName = internalCluster().getMasterName();
         final DiscoveryNode preferredMaster = internalCluster().clusterService(preferredMasterName).localNode();

test/framework/src/main/java/org/elasticsearch/test/InternalTestCluster.java

@@ -1756,7 +1756,8 @@ public final class InternalTestCluster extends TestCluster {
 
 
     public void setDisruptionScheme(ServiceDisruptionScheme scheme) {
-        clearDisruptionScheme();
+        assert activeDisruptionScheme == null :
+            "there is already and active disruption [" + activeDisruptionScheme + "]. call clearDisruptionScheme first";
         scheme.applyToCluster(this);
         activeDisruptionScheme = scheme;
     }