standardize tests: template macros, c++20 ranges, naming conventions

[cameroncuster]

Standardizes tests/library_checker_aizu_tests/ to the same conventions the library already follows: template macros, C++20 ranges/<bit>, and the naming conventions from #223. Test-only change — no library headers touched.

Tests already include kactl_macros.hpp (via template.hpp), so the macros were available everywhere but used inconsistently — e.g. 14 files used rep(...) while 108 used raw loops, and 84 files used vector<int> while 31 used vi, sometimes mixed within one file.

Template macros (matching library style)

• for (int i = a; i < b; i++) -> rep(i, a, b) (~290 sites; loops with a second init variable, non-< conditions, or non-++ increments left as raw for)
• vector<int> -> vi (175 sites), pair<int, int> -> pii (28)
• long long / int64_t -> ll (33 sites; uint64_t untouched)
• ssize(x) / (int)size(x) -> sz(x) (20)
• begin(x), end(x) -> all(x) (47)

C++20 standardization (matching #221)

Use the ranges::/views::/<bit> form consistently wherever it applies:

• full-range algo(all(x), ...) -> ranges::algo(x, ...) for sort, reverse, fill, count, find, min/max_element, generate, transform, lower_bound, etc. (~70 sites across 29 files)
• membership checks use ranges::find(x, v) == end(x) / ranges::count; associative containers use .contains()
• __lg -> <bit> (bit_width/bit_floor) in seg_tree_midpoint.test.cpp
• left as-is: iota, accumulate, partial_sum (no C++20 ranges counterpart), genuinely offset/partial ranges
• left as-is: unique/remove_if stay in iterator form (algo(all(x), ...)) — the ranges:: versions return a subrange needing a begin(...) unwrap and trip cppcheck mismatchingContainerExpression false positives (same rule as standardize on c++20: ranges/views and <bit> used consistently #221)

Naming conventions (matching #223)

• ans -> res (range_parallel_dsu, count_paths)
• curr -> cur, prev -> prv (5 files)
• seen/used -> vis (3 files)
• priority -> pri (pq_ds_undo_sliding_window)
• next -> nxt (functional_graph, directed_cycle)

Verification

• clang-format --dry-run --Werror (dev config) passes on all touched files
• all changed test files compile with g++ -std=c++20 -O2 -Wall -Wextra
• repo grep checks pass: no endl, no member-call .size()/.begin() forms, no long long/vector<int> stragglers, no __lg
• cppcheck suppression list audited: all line-pinned test-file entries still point at the same statements (the suppressed lines did not shift); several entries reference files that no longer exist, which predates this PR