Run unit tests and integration tests together to speed up PR builds

[agriyakhetarpal]

Description

The PR builds run unit tests and integration tests separately right now, which leads to a lot of runner requests from GitHub Actions (more than what are available to us concurrently with a non-team or non-enterprise plan), and as a result they usually take up to an hour to complete. A way to improve the PR build times would be to run nox -s unit and nox -s integration together till the time #3617 is finished and the integration tests are migrated to support pytest invocations, because the unit tests after #3857 run in parallel, and usually pass within ~5-6 minutes across platforms (and are much faster within the range of <3 minutes on larger runners such as the macos-14 ones).

Proposed implementation

Modify test_on_push.yml to run nox -s unit and nox -s integration in the same step, remove one (either) of the redundant unit or the integration jobs, and adjust the job names as necessary.

Additional context

Discussed in the community bonding GSoC meeting on 17/05/2024 with @Saransh-cpp and @arjxn-py – cc: @prady0t (you'll have to comment on the issue in order for us to assign you to this)

[prady0t]

I'll start working on this right away. You can assign this to me.

[agriyakhetarpal]

Adding xrefs: #3662