test!: migrate custom check() suite to testthat 3e

[davidbudzynski]

Closes #54.

Problem

tests/test_kit.R:7-34 defined check(test,x,y,error,warning) that only did cat("Check ... failed.\n") with no stop()/stopifnot(). Result: R CMD check always green even with regressions (1286 silent checks).

What this PR does

• DESCRIPTION: add Config/testthat/edition: 3 + Suggests: testthat (>= 3.0.0)
• New runner tests/testthat.R (test_check("kit"))
• Split suite into tests/testthat/:
  • test-topn.R, test-iif-nif.R, test-parallel.R (psum/pprod/fpmin/fpmax/prange/pall/pany/pmean), test-share.R (as requested in test!: custom check() never fails - migrate to testthat3 #54), plus test-fpos.R, test-setlevels.R, test-vswitch-nswitch.R, test-count.R, test-unique.R, test-psort.R, test-charToFact.R, test-pfirst-plast.R, helper-kit.R
• Mapping:
  • check(id, expr, expected) -> expect_identical()
  • 31 tolerant cases where identical() is FALSE but legacy all.equal()+typeof() is TRUE (complex NA imaginary NA vs 0, C-vs-R floating-point order in pmean/pprod/vswitch) -> expect_kit_equal() helper (mirrors legacy logic via expect())
  • check(..., error=) -> expect_error(regexp=, fixed=TRUE) (mirrors legacy grep fixed=TRUE substring match)
  • 2 warning cases (0001.068, 0020.001) -> expect_warning(..., fixed=TRUE) + value comparison
• Legacy IDs preserved as test_that() labels, function-prefixed for uniqueness (topn-0001.001, vswitch-0008.001 vs fpmin-0008.001, etc.). Duplicate legacy IDs (0002.101, 0016.065) get -2 suffix.
• Setup and expectations stay interleaved in legacy order (later sections redefine x/y/out_vec, so setup is not hoisted).
• Delete legacy tests/test_kit.R (replaced by runner). 17 commented-out psort checks from legacy file are omitted (were commented).
• test-coverage.yaml: no change needed — verified remotes::install_deps(dependencies=TRUE) already picks up Suggests: testthat and covr::codecov() auto-detects tests/testthat.

Verification

• test_dir("tests/testthat"): 1271 expectations, 0 failures (1268 test_that blocks covering 1269 legacy checks).
• R CMD check --no-manual --no-vignettes: Running 'testthat.R' ... OK (only pre-existing vignette inst/doc warnings from --no-build-vignettes).
• Mutation check: temporarily changed topn-0001.001 expected to 999L -> suite fails as expected (Expected topn(...) to be identical to 999L / actual: 5), then restored. Old suite would only cat() and exit 0.

Notes for reviewers

• expect_kit_equal() is intentionally narrow (31 cases). If we prefer strict waldo for those too, we would need to update expected values (e.g. complex NA imaginary) — left as legacy-tolerant for a green baseline.
• Happy to split further or rename files if you prefer 1-file-per-function.