Add test template

[mreso]

Description

This PR add an easy integration test template using mnist and documents running tests in the CONTRIBUTION.md

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Feature/Issue validation/testing

Please describe the Unit or Integration tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

• pytest -k test_mnist_template test/pytest/test_mnist_template.py

====================================================================================================================================== test session starts =======================================================================================================================================
platform linux -- Python 3.11.5, pytest-7.3.1, pluggy-1.0.0
rootdir: /home/ubuntu/serve
plugins: mock-3.12.0, cov-4.1.0
collected 1 item

test/pytest/test_mnist_template.py 2024-05-09T20:30:52,585 [INFO ] W-9000-some_model_1.0 TS_METRICS - QueueTime.Milliseconds:0.0|#Level:Host|#hostname:ip-172-31-55-226,timestamp:1715286652
2024-05-09T20:30:52,585 [INFO ] W-9000-some_model_1.0 org.pytorch.serve.wlm.WorkerThread - Backend response time: 8
2024-05-09T20:30:52,586 [INFO ] W-9000-some_model_1.0 TS_METRICS - WorkerThreadTime.Milliseconds:2.0|#Level:Host|#hostname:ip-172-31-55-226,timestamp:1715286652
.                                                                                                                                                                                                                                                       [100%]

======================================================================================================================================= 1 passed in 5.89s ========================================================================================================================================

Checklist:

• Did you have fun?
• Have you added tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?