libc: fix setvbuf_fullbuffer_overflow testcase

[damianloew]

Description

Motivation and Context

The test case checks fully buffered mode. Previous implementation required data overflowing the buffer (sending 10 bytes when a buffer is 8 bytes long) to be written to a file at once which is not required by POSIX. The current solution forces flushing the remaining 2 bytes anyway

JIRA: CI-507

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: (ia32-generic-qemu with https://github.com/phoenix-rtos/libphoenix/pull/376/commits changes applied, host-pc).

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 (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

7 741 tests  ±0   7 026 ✅ ±0   41m 20s ⏱️ + 1m 47s
  461 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 43b4d23. ± Comparison against base commit 0733d9f.

[adamdebek]

At this moment, this testcase is tailored to pass in glibc and doesn't check if buffering works correctly on phoenix. I would make this testcase phoenix specific.

[ziemleszcz]

LGTM

[damianloew]

pass in glibc

Yeah, but then we would test characteristics that are implementation-defined. In those tests we focus on satisfying POSIX requirements which only requires those 8 bytes to be written without flush. Also, it would be much simpler to merge those 'flexible' changes as we accept an older Phoenix implementation.