Use mock data for element operator tests.

[ChiahungTai]

Description

Use mock data for element operator tests.

Motivation and Context

To speed up the operator test and give the freedom opertor test, the use of mock data is more convinent.

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/ops/test_elem_operator.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:

Types of changes

• Fix bugs
• Add new feature
• Update documentation
• Add new test cases

[ChiahungTai]

@you-n-g,
Could you please review this patch?
I would like to add more test cases for the other opertors based on this patch.
Thanks.

[you-n-g]

Thanks for your contribution.
I think it is quite helpful!
It will be easier and clearer to write tests now :)