Fix flaky boltkit test

`GraphDatabaseTest#boltSchemeShouldInstantiateDirectDriver()` did not
properly close driver which resulted in sporadically failing Windows builds.

Author: lutovich

driver/src/test/java/org/neo4j/driver/v1/GraphDatabaseTest.java

@@ -50,7 +50,10 @@ public void boltSchemeShouldInstantiateDirectDriver() throws Exception
 
         // Then
         assertThat( driver, is( directDriver() ) );
-        server.exit();
+
+        // Finally
+        driver.close();
+        assertThat( server.exitStatus(), equalTo( 0 ) );
     }
 
     @Test

driver/src/test/java/org/neo4j/driver/v1/util/StubServer.java

@@ -79,7 +79,7 @@ public int exitStatus() throws InterruptedException, ForceKilled
         }
     }
 
-    public void exit() throws InterruptedException
+    private void exit() throws InterruptedException
     {
         process.destroy();
         process.waitFor();