docs(core): drop restatement comments in tests across core

[oferchen]

Summary

Per-crate comment cleanup pass on the core crate, following the same precedent as PRs #4591 (protocol), #4590 (transfer), #4589 (fast_io), #4584 (filters), #4582 (logging), #4583 (platform), #4587 (matching), #4575 (compress).

The core crate was already in good shape thanks to its #![deny(missing_docs)] policy:

• All public types, traits, functions, and re-exports carry rustdoc (verified by the crate-level deny).
• Production code is dominated by upstream cites (main.c, options.c, compat.c, flist.c, rsync.c, clientserver.c, util1.c, log.c) and WHY notes covering iconv apply hook ordering, INC_RECURSE gating, local-copy progress assumptions, transport-selection rationale, and shell quoting rules - all preserved.
• The signal/ subtree was excluded from this pass per the BR-10b precedent (PR fix(core): move sigaction unsafe out of core into fast_io #4571); its docs are already accurate.

Net change: 69 lines removed, 1 added across 12 files. Removals target:

• Test-section header banners (// Tests for X) that simply prefix a group of #[test] functions whose names already describe the subject.
• Comments that restate the next line of code or the immediately following assertion message verbatim.
• A handful of stray "comment that says what the variable is" annotations in test helpers.

Scope coverage

Walked the entire crates/core/src/ tree except signal/ (excluded per task scope). No scope cap applied - the crate was already clean enough that the full walk fit comfortably within one PR.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --no-deps -- -D warnings
• CI matrix (fmt+clippy, nextest, Windows, macOS, Linux musl)