libc/misc: add tests for libphoenix __safe_read/write functions

[lukileczo]

JIRA: RTOS-882

Description

Add simple tests for libphoenix internal __safe_read/write functions.

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work unistd/file: fix __safe_read/write return value libphoenix#372
• I will merge this PR by myself when appropriate.

[agkaminski]

LGTM, perhaps we could also add __safe_open and __safe_close to the mix

[github-actions]

Unit Test Results

7 700 tests   6 977 ✅  37m 42s ⏱️
  436 suites    715 💤
    1 files        8 ❌

For more details on these failures, see this check.

Results for commit bb1be92.

♻️ This comment has been updated with latest results.

[lukileczo]

LGTM, perhaps we could also add __safe_open and __safe_close to the mix

Done