refactor(test): use <+ template writing in test files

[bobzhang]

Part 1 of 2, split out of #4110. Test and benchmark files only — no
library code is touched, and no expected output changes.

18 files, −85 / +52 lines.

What changes

• buf.write_string("...\{x}...") → buf <+ "...\{x}...". The old form built
  a throwaway StringBuilder + String for the interpolation and then copied
  the result into buf; the template writes the pieces straight through.

• Runs of writes collapse into one template:

  arr.rev_eachi((i, x) => {
    buf.write_object(i)
    buf.write_string(": ")
    buf.write_object(x)
    buf.write_string("\n")
  })
  // becomes
  arr.rev_eachi((i, x) => buf <+ "\{i}: \{x}\n")

  Adjacent literal chunks are concatenated by the desugarer at compile time,
  so the literal part of a run becomes a single write_string.

Note on the hole form

These use the plain \{x} hole, which desugars to
write_string_interpolation(x) and coerces x to &Show — one box per hole.
That is the right trade in a three-element assertion, but not in library code,
which is why #4110 uses the inline-writer form \{l => l.write_object(x)}
there instead. The reasoning and benchmarks for that choice live in #4110.

Scope boundary

Strictly *_test.mbt / *_wbtest.mbt files. builtin/fixedarray.mbt also has
a <+ rewrite confined to an in-file test {} block, but it is a library file,
so it stays in #4110 rather than here.

Verification

• moon check --deny-warn --target all
• moon test on wasm / wasm-gc / js / native — 7458 / 7459 / 7403 / 7375
  passed, 0 failed
• moon fmt clean

[Copilot]

Pull request overview

Refactors MoonBit test and benchmark files to use the <+ template-writing operator instead of write_string / write_char / write_object sequences (and string-interpolation passed into write_string), reducing intermediate string allocations while preserving existing snapshot/expected outputs.

Changes:

• Replace buf.write_string("...\{x}...")-style interpolation (which materializes an intermediate string) with buf <+ "...\{x}..." in tests/benches.
• Collapse multi-call write sequences into single <+ templates (often adding \n literals directly in the template).
• Minor formatting adjustments to keep the test output identical while simplifying write logic.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
strconv/quickcheck_test.mbt	Use <+ templates to build structured numeric strings without intermediate to_string() writes.
strconv/double_differential_test.mbt	Collapse per-line output assembly into single <+ templates for probe rendering.
sorted_set/set_test.mbt	Replace per-element interpolated write_string with <+ in iteration output.
sorted_map/map_test.mbt	Use <+ templates for map iteration formatting in tests.
set/linked_hash_set_test.mbt	Convert iterator element formatting to <+ templates.
priority_queue/priority_queue_test.mbt	Use <+ templates for priority queue iteration formatting.
option/option_test.mbt	Replace multi-step newline writing with "<+ \"\\{x}\\n\"" in iterator tests.
json/number_bench_test.mbt	Collapse float-number JSON element writes into a single <+ template.
immut/vector/vector_test.mbt	Convert vector iteration formatting to <+ templates.
immut/sorted_map/utils_test.mbt	Collapse key/value formatting sequences into <+ templates (including Repr cases).
immut/sorted_map/map_test.mbt	Replace interpolated write_string calls with <+ in iterator tests.
immut/priority_queue/priority_queue_test.mbt	Convert iteration formatting to <+ templates in immut priority queue tests.
deque/deque_test.mbt	Replace to_string() + newline writes with a single <+ template.
builtin/stringview_test.mbt	Use <+ for per-iteration formatted output in core StringView test.
builtin/linked_hash_map_wbtest.mbt	Convert debug-entry formatting to <+ template within whitebox tests.
builtin/iter_test.mbt	Use <+ templates for mapped iterator output with newlines.
builtin/array_test.mbt	Collapse rev_eachi write sequences into a single <+ template.
bigint/bigint_nonjs_wbtest.mbt	Switch Show output implementation in wbtest fixture to <+ template writing.

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[bobzhang]

Split from #4110. This half is test-only and can land independently; #4110 now carries the library changes and is the one that needs review.

[coveralls]

Coverage Report for CI Build 6200

Coverage remained the same at 90.718%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	17960
Covered Lines:	16293
Line Coverage:	90.72%
Coverage Strength:	332058.84 hits per line

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💛 - Coveralls