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ARROW-7899: [Integration][Java] Fix Flight integration test client to verify each batch

Previously the Java Flight integration test client would read a number of batches from the server, but only verify the last one is equal the last batch in the JSON file. This fixes the client to verify each batch is correct and that the number of batches sent is the same as in the JSON input file.

Closes #6476 from BryanCutler/integration-null-test-ARROW-7899 and squashes the following commits:

6d1cfc3 <Bryan Cutler> Change Java Flight test client to not error on trailing empty batches
fcc7390 <Bryan Cutler> Fix Flight integration test client to verify each batch

Authored-by: Bryan Cutler <cutlerb@gmail.com>
Signed-off-by: Bryan Cutler <cutlerb@gmail.com>