Improve coverage of equals/hashCode tests for IntervalQueryBuilder (#49820)

By default, AbstractQueryTestCase only changes name and boost in its mutateInstance
method, used when checking equals and hashcode implementations. This commit adds
a mutateInstance method to InveralQueryBuilderTests that will check hashcode and
equality when the field or intervals source are changed.

server/src/main/java/org/elasticsearch/index/query/IntervalQueryBuilder.java

@@ -55,6 +55,14 @@ public class IntervalQueryBuilder extends AbstractQueryBuilder<IntervalQueryBuil
         this.sourceProvider = in.readNamedWriteable(IntervalsSourceProvider.class);
     }
 
+    public String getField() {
+        return field;
+    }
+
+    public IntervalsSourceProvider getSourceProvider() {
+        return sourceProvider;
+    }
+
     @Override
     protected void doWriteTo(StreamOutput out) throws IOException {
         out.writeString(field);

server/src/test/java/org/elasticsearch/index/query/IntervalQueryBuilderTests.java

@@ -143,6 +143,17 @@ public class IntervalQueryBuilderTests extends AbstractQueryTestCase<IntervalQue
         assertThat(query, instanceOf(IntervalQuery.class));
     }
 
+    @Override
+    public IntervalQueryBuilder mutateInstance(IntervalQueryBuilder instance) throws IOException {
+        if (randomBoolean()) {
+            return super.mutateInstance(instance); // just change name/boost
+        }
+        if (randomBoolean()) {
+            return new IntervalQueryBuilder(STRING_FIELD_NAME_2, instance.getSourceProvider());
+        }
+        return new IntervalQueryBuilder(STRING_FIELD_NAME, createRandomSource(0));
+    }
+
     public void testMatchInterval() throws IOException {
 
         String json = "{ \"intervals\" : " +