ARROW-7713: [Java] TastLeak was put at the wrong location

Related to [ARROW-7713](https://issues.apache.org/jira/browse/ARROW-7713).

Seems TestLeak.java was put at the wrong place, we should move it into flight-core.

Closes apache#6311 from tianchen92/ARROW-7713 and squashes the following commits:

d04fbc2 <tianchen> ARROW-7713:  TastLeak was put at the wrong location

Authored-by: tianchen <niki.lj@alibaba-inc.com>
Signed-off-by: David Li <li.davidm96@gmail.com>

Author: tianchen92

java/flight/src/test/java/org/apache/arrow/flight/TestLeak.java renamed to java/flight/flight-core/src/test/java/org/apache/arrow/flight/TestLeak.java

@@ -29,7 +29,6 @@
 import org.apache.arrow.vector.types.pojo.ArrowType;
 import org.apache.arrow.vector.types.pojo.Field;
 import org.apache.arrow.vector.types.pojo.Schema;
-
 import org.junit.Test;
 
 /**