Modularized test_main tests

[oleksandr-pavlyk]

Also added code to work around a bug with CPU runtime, which may cause DPC++ RT to not see CPU device in the sub-process if the parent process has initialized CPU plugin.

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• If this PR is a work in progress, are you opening the PR as a draft?

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[coveralls]

coverage: 91.244%. remained the same
when pulling 7a6a1bf on change-test-main-to-work-around-a-problem
into d2d06fd on master.

[github-actions]

Array API standard conformance tests for dpctl=0.17.0dev0=py310h15de555_43 ran successfully.
Passed: 904
Failed: 2
Skipped: 94

[ndgrigorian]

LGTM!