Run individual pytests instead of running all pytests at once.

[mint570]

What I did
Run pytests one test at a time.

Why I did it
Fix #2330

How I verified it

Details if related

[mint570]

@pettershao-ragilenetworks

[mint570]

@lguohan

[mint570]

@prsunny @pettershao-ragilenetworks
Can you please take a look at this PR?

[mint570]

@theasianpianist can you please take a look?

[prsunny]

@theasianpianist , please review

[prsunny]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[prsunny]

lgtm, @liushilongbuaa , @xumia , could you please review?

[liushilongbuaa]

LGTM

[prsunny]

@mint570 , as discussed offline with @theasianpianist , we see a significant increase in time for the test completion.

Can you please work on an alternative solution for #2373 and #2330?

[mint570]

I don't have an obvious solution now. The increase in time is mostly due to the overhead of running individual test.
There might be some improvement we can do. Such as grouping the tests together (instead of running 100 tests, we can run 10 test groups, each includes 10 tests). But the overall time will still be increased compare to running all tests at once.

[mint570]

Some random ideas. May or may not help:

1. Integrate the coverage file collection into the pytest. So that we can run all tests in one pytest call.
2. Share a single vs container with all tests.

Not sure if the above is possible or not. I tried to look at how the pytest dvs works. But unfortunately, I don't have enough expertise to understand that.