Drop bash 5 assumptions from the test harness

[jserv]

Makefile invokes scripts via plain "bash tests/foo.sh", so stock macOS picks up /bin/bash 3.2.57 regardless of the env-bash shebang. 2 failures surfaced on a clean Apple Silicon host without Homebrew bash, both under set -u:

• tests/lib/test-runner.sh:55 read $EPOCHREALTIME (bash 5.0+ builtin), which expanded to the empty string and tripped the unbound-variable check. make test-busybox aborted at the first sample.
• tests/driver.sh did "unset 'argv[0]'" then expanded "${argv[@]}". bash 3.2 reports the emptied array as unbound; the fix in 4.4+ silently returns zero elements. make check aborted partway through.

Land a shared tests/lib/bash-compat.sh that consolidates the cross-version shims so each script does not have to re-derive them:
bash_compat_require aborts with rc=127 and a brew-install hint if
BASH_VERSINFO is missing or below 3.2. The minimum is parameterised
through BASH_COMPAT_MIN_MAJOR / BASH_COMPAT_MIN_MINOR so a future bump
is a single-line change.

epoch_us prints integer microseconds. It picks the lowest-cost source
at sourcing time: $EPOCHREALTIME on bash 5.0+ (no fork), "date +%s %N"
when the probe shows the system date emits a 9-digit nanosecond field
(GNU coreutils and macOS 14+/Sonoma date both qualify), python3, perl,
or a whole-second fallback. The chosen implementation is bound to one
function name so callers stay version-agnostic.

The file header documents the supported subset: no EPOCHREALTIME, no
declare -A, no mapfile / readarray, no ${var^^} / ${var,,}, and guard
any potentially empty array expansion with ${arr[@]+"${arr[@]}"} so
set -u does not trip on it.

Wire the shim through the four hot scripts:
tests/lib/test-runner.sh sources bash-compat.sh and uses epoch_us in
the run() watchdog. The four "${TEST_RUNNER[@]}" expansions gain the
${TEST_RUNNER[@]+...} guard so the library is self-defending if a
caller ever sources it without setting the array first.

tests/driver.sh drops the "unset argv[0]" dance for an offset slice
"for arg in "${argv[@]:1}"", which produces zero iterations cleanly
even when argv has one element. The downstream "${args[@]}" expansion
picks up the same +guard so tests with no extra args still launch.

tests/test-perf.sh drops its local epoch_us in favor of the shared
helper and sources bash-compat.sh.

tests/test-rosetta-alpine.sh guards args_a / args_b which could be
empty when a pipeline test omits one side of the "--" separator.

Refactor the two "declare -A" uses to indexed-array + lookup pattern:
tests/fetch-fixtures.sh encodes PKGS as "repo:name:version" tuples and
exposes a pkg_version helper. The case "$entry" in "$target") form
with quoted target is glob-safe ("" and "?" in target match
literally) and the trailing ":" prevents prefix collisions
(main:bash does not match main:bash-completion).

tests/test-matrix.sh encodes EXPECTED_BASELINES as "key |min|max" and
exposes expected_baseline_get with printf -v output parameters.
Parsing is right-anchored (${entry%|} then ${head##|}) so a key
containing a literal "|" would still split correctly; current keys
have none but the form removes the trap. The verify_expected_counts
call site flips to "if ! expected_baseline_get" so a miss stays
silent the way the prior "${arr[$key]:-}" empty-string check did.

Close #66

---

Summary by cubic

Make the test harness run on stock macOS /bin/bash 3.2 by removing bash 5-only features and adding a shared compatibility shim. Fixes set -u errors, hardens array handling, and keeps performance timing precise without requiring Homebrew bash.

• Bug Fixes

  • Address set -u failures on bash 3.2: replace $EPOCHREALTIME, guard empty array expansions, and switch tests/driver.sh to "${argv[@]:1}" instead of unsetting argv[0].
  • Guard ${TEST_RUNNER[@]}/${args[@]} expansions across runners; Rosetta pipeline now handles empty sides safely.

• Refactors

  • Add tests/lib/bash-compat.sh with bash_compat_require and epoch_us (prefers $EPOCHREALTIME, falls back to date +%s %N, python3, or perl) and source it from tests/lib/test-runner.sh and tests/test-perf.sh.
  • Replace associative arrays with portable indexed arrays: PKGS + pkg_version in tests/fetch-fixtures.sh, EXPECTED_BASELINES + expected_baseline_get in tests/test-matrix.sh.
  • Update docs/testing.md to state bash 3.2+ is sufficient and document the portable bash rules (no associative arrays, no ${var^^}/${var,,}, guard empty array expansions, etc.).

^{Written for commit 018857e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 8 files

_{Re-trigger cubic}