Fix broken explain tests

JAVA-3909

Author: jyemin

driver-core/src/test/functional/com/mongodb/internal/operation/AggregateOperationSpecification.groovy

@@ -339,6 +339,7 @@ class AggregateOperationSpecification extends OperationFunctionalSpecification {
         async << [true, false]
     }
 
+    @IgnoreIf({ !serverVersionAtLeast(3, 6) })
     def 'should be able to explain an empty pipeline'() {
         given:
         def operation = new AggregateOperation(getNamespace(), [], new BsonDocumentCodec())
@@ -384,7 +385,7 @@ class AggregateOperationSpecification extends OperationFunctionalSpecification {
 
         when:
         execute(operation, async)
-        BsonDocument explainPlan = execute(operation.asExplainableOperation(QUERY_PLANNER), async)
+        BsonDocument explainPlan = execute(operation.asExplainableOperation(QUERY_PLANNER, new BsonDocumentCodec()), async)
 
         then:
         getKeyPattern(explainPlan.getArray('stages').get(0).asDocument().getDocument('$cursor')) == index

driver-legacy/src/test/functional/com/mongodb/DBCollectionAggregationTest.java

@@ -106,14 +106,10 @@ public List<DBObject> prepareData() {
 
     @Test
     public void testExplain() {
+        assumeTrue(serverVersionAtLeast(3, 6));
         List<DBObject> pipeline = new ArrayList<DBObject>(prepareData());
-        pipeline.add(new BasicDBObject("$out", "aggCollection"));
-        CommandResult out = collection.explainAggregate(pipeline, AggregationOptions.builder()
-            .allowDiskUse(true)
-            .build());
-        assertTrue(out.keySet()
-                      .iterator()
-                      .hasNext());
+        CommandResult out = collection.explainAggregate(pipeline, AggregationOptions.builder().build());
+        assertTrue(out.keySet().iterator().hasNext());
     }
 
     @Test(expected = IllegalArgumentException.class)

driver-sync/src/test/functional/com/mongodb/client/AbstractExplainTest.java

@@ -124,6 +124,7 @@ public void testExplainOfAggregateWithNewResponseStructure() {
     public void testExplainOfAggregateWithOldResponseStructure() {
         // Aggregate explain is supported on earlier versions, but the structure of the response on which we're asserting in this test
         // changed radically in 4.2. So here we just assert that we got a non-error respinse
+        assumeTrue(serverVersionAtLeast(3, 6));
         assumeTrue(serverVersionLessThan(4, 2));
 
         MongoCollection<BsonDocument> collection = client.getDatabase(getDefaultDatabaseName())