IGNITE-14007 Fixes unclosed Ignite instance in Flink integration tests. (#36)

diff --git a/modules/flink-ext/src/test/java/org/apache/ignite/sink/flink/FlinkIgniteSinkSelfTest.java b/modules/flink-ext/src/test/java/org/apache/ignite/sink/flink/FlinkIgniteSinkSelfTest.java
index 14798ed..ef7ce2a 100644
--- a/modules/flink-ext/src/test/java/org/apache/ignite/sink/flink/FlinkIgniteSinkSelfTest.java
+++ b/modules/flink-ext/src/test/java/org/apache/ignite/sink/flink/FlinkIgniteSinkSelfTest.java
@@ -22,6 +22,8 @@
 import org.apache.flink.configuration.Configuration;
 import org.apache.flink.streaming.api.datastream.DataStream;
 import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
+import org.apache.ignite.Ignition;
+import org.junit.AfterClass;
 import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
@@ -37,6 +39,12 @@
     /** Ignite test configuration file. */
     private static final String GRID_CONF_FILE = "config/example-ignite.xml";
 
+    /** */
+    @AfterClass
+    public static void afterTests() {
+        Ignition.stopAll(true);
+    }
+
     @Test
     public void testIgniteSink() throws Exception {
         Configuration configuration = new Configuration();