testsuite: fix flaky byte-corruption in check_cmd tests

[ThomasWaldmann]

What

Several tests in archiver/check_cmd_test.py intermittently failed (e.g. test_spoofed_archive with assert 0 == 1 on the first check).

Why

They corrupted a repo object by overwriting a byte at a fixed position with a fixed value:

corrupted_manifest = manifest[:250] + b"x" + manifest[251:]

Manifests and chunks are stored as AEAD-encrypted repo objects (repokey-aes-ocb), so their bytes are essentially random and differ every run. Whenever the targeted byte already happened to hold the overwrite value (~1 in 256), the "corruption" was a no-op: the object stayed valid, check found nothing wrong and returned 0, and the exit_code=1 assertion failed — hence the flakiness.

Fix

Introduce a small helper that flips the byte (XOR 0xFF), so the result is guaranteed to differ:

def corrupt(data, position):
    if position < 0:
        position += len(data)
    return data[:position] + bytes([data[position] ^ 0xFF]) + data[position + 1 :]

and use it at all the byte-overwrite corruption sites: test_corrupted_manifest, test_spoofed_manifest, test_manifest_rebuild_corrupted_chunk, test_spoofed_archive, test_verify_data, test_corrupted_file_chunk.

The crypto unit tests (crypto_test.py) do a similar overwrite but with fixed keys/IVs and deterministic output (they assert exact MAC hex), so they are not affected and are left unchanged.

Test

Ran the affected tests locally: 14 passed, 10 skipped (skips are the remote/binary variants).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.90%. Comparing base (e0a040a) to head (27401aa).
⚠️ Report is 6 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9771      +/-   ##
==========================================
+ Coverage   84.86%   84.90%   +0.04%     
==========================================
  Files          92       92              
  Lines       15172    15107      -65     
  Branches     2273     2260      -13     
==========================================
- Hits        12875    12826      -49     
+ Misses       1593     1581      -12     
+ Partials      704      700       -4     

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