Convert tests/kernel/workq to new kwork API

[galak]

Convert tests/kernel/workq/critical/src/main.c & tests/kernel/workq/work_queue/src/main.c to use new kwork API.

See the following issue for details:

#33104

[pabigot]

Two choices for the maintainer:

• (Recommended) Once all other in-tree uses are converted merge kernel: deprecate old k_work API #32888 to deprecate the old API. That PR includes a solution that retains the tests of the deprecated API without causing CI failures. tests: kernel: workq: update for removal of deprecated API #34112 should be merged when the deprecated API is removed in a future release.
• (Not Recommended) Merge tests: kernel: workq: update for removal of deprecated API #34112 immediately, opening the possibility that the behavior of the deprecated API will be broken before the API is removed.

[github-actions]

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