CI: move [ocaml-check] and [check_riscv_kernels] in [test] stage

What

• Move [ocaml-check] and [check_riscv_kernels] jobs from [build] to [test] stage

Why

• These jobs do not build but test.
• Furthermore, for [check_riscv_kernels], it will avoid depending on a job in a later stage, as it depends on [audit_riscv_kernels] that we want to move from [sanity] to [test]. In other words, this MR is a prerequisite for another MR that will remove the [sanity] stage.

How

• change stage + make -C ci

Manually testing the MR

• make -C ci check
• checks that a valid before_merging pipeline is created for this MR: https://gitlab.com/tezos/tezos/-/pipelines/1876018078

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR