Enforce comparison test in new SQL module

[dai-chen]

Issue #, if available: #535

Description of changes: Last year we developed a test framework which verifies result by comparing with other SQL implementation. It was integrated with Gradle task to automate the testing. However, due to problems in old codebase, it was only used for reporting instead of being mandatory for plugin build. Now as we start shift to new query engine, the success of comparison test results should be enforced. Reference: https://github.com/opendistro-for-elasticsearch/sql/blob/develop/docs/dev/Testing.md.

1. Add SQL test base class and a new IT class SQLCorrectnessIT to read all queries in specific folder.
2. Improve test framework by explaining the difference that caused the test case failure.

For now test case file is simply a txt file organized in folders under src/resources/correctness in integ-test module. There are 3 categories:

1. expressions: test cases for all expressions such as arithmetic, predicate and function expressions.
2. queries: test cases for different queries such as SELECT-FROM-WHERE, GROUP BY, JOINs etc.
3. bugfixes: test cases for bug fix to make sure no regression in future release.

$ tree -L 2 -I "resources" integ-test/src/test/resources/correctness
integ-test/src/test/resources/correctness
├── bugfixes
│   └── 521.txt
├── expressions
│   ├── arithmetics.txt
│   ├── functions.txt
│   ├── literals.txt
│   └── predicates.txt
├── queries
│   ├── groupby.txt
│   ├── joins.txt
│   ├── select.txt
│   └── subqueries.txt
├── kibana_sample_data_ecommerce.csv
├── kibana_sample_data_ecommerce.json
├── kibana_sample_data_flights.csv
└── kibana_sample_data_flights.json

For example, for failed test case, a new explain field is added to describe which column type or data row is different.

{
   "summary":{
      "total":1,
      "success":0,
      "failure":1
   },
   "tests":[
      {
         "result":"Failed",
         "explain":"Data row at [0] is different: this=[Row(values=[hello])], other=[Row(values=[world])]",
         "resultSets":[
            {
               "schema":[
                  {
                     "name":"firstName",
                     "type":"text"
                  }
               ],
               "dataRows":[
                  [
                     "hello"
                  ]
               ],
               "database":"Elasticsearch"
            },
            {
               "schema":[
                  {
                     "name":"firstName",
                     "type":"text"
                  }
               ],
               "dataRows":[
                  [
                     "world"
                  ]
               ],
               "database":"H2"
            }
         ],
         "id":1,
         "sql":"SELECT * FROM accounts"
      }
   ]
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[penghuo]

LGTM.

[chloe-zh]

LGTM, thanks!