[Intermediate Representation] Add e2e test for IR analyze

[weiguoz]

Fix #977

[Yancey0623]

LGTM with tiny comment.

[typhoonzero]

Please note that go test may run test cases unordered, the model may be not generated when running this test, it's better to generate this model first before running ANALYZE.

[weiguoz]

The ref introduces a go test run parallel with other tests if and only if the t.Parallel() was called.

[typhoonzero]

This is not about parallelism, it's about the order, see https://stackoverflow.com/questions/31201858/how-to-run-golang-tests-sequentially, it says:

You can't / shouldn't rely on test execution order. The order in which tests are executed is not defined, and with the use of testing flags it is possible to exclude tests from running, so you have no guarantee that they will run at all.

[weiguoz]

In our e2e test, the subtests are ordered by group

sqlflow/cmd/sqlflowserver/main_test.go

Lines 308 to 310 in 672d4ef

	t.Run("CaseTrainXGBoostRegressionIR", CaseTrainXGBoostRegression)
	t.Run("CasePredictXGBoostRegressionIR", CasePredictXGBoostRegression)
	t.Run("CaseAnalyzeXGBoostModel", CaseTrainAndAnalyzeXGBoostModel)

But we would run go test -run CaseTrainAndAnalyzeXGBoostModel independently some times, it needs an existed model, so I add the several codes for that.