tests.yml: install i386 runtime libs before setup-julia for 32-bit legs

[ChrisRackauckas-Claude]

Second half of enabling 32-bit test lanes (first half — the arch axis + group alias — shipped in #113 / v1.24.0).

Problem

A 32-bit leg (a test_groups.toml group with arch = "x86", forwarded as julia-arch: x86) fails at the Setup Julia step on the 64-bit Linux runners:

spawn /home/runner/_work/_tool/julia/1.12.6/x86/bin/julia ENOENT

The i686 Julia binary downloads fine, but it can't exec without the i386 dynamic loader / C++ runtime, which ubuntu-latest lacks. setup-julia invokes julia during its own step, so an apt step later in the job is too late.

Fix

Add a step before setup-julia, gated on a 32-bit julia-arch on Linux, that installs the i386 runtime:

- name: "Install 32-bit runtime libraries (i386)"
  if: "${{ (inputs.julia-arch == 'x86' || inputs.julia-arch == 'i686') && runner.os == 'Linux' }}"
  run: |
    sudo dpkg --add-architecture i386
    sudo apt-get update
    sudo apt-get install -y --no-install-recommends \
      libc6:i386 libstdc++6:i386 libgcc-s1:i386 libatomic1:i386

Non-32-bit legs are untouched (the if is false — no extra time).

Verification

In a clean ubuntu:24.04 container (matching ubuntu-latest):

• Before the libs: julia-1.12.6/bin/julia: cannot execute: required file not found (the ENOENT).
• After apt install libc6:i386 …: WORD_SIZE=32 ARCH=i686 VERSION=1.12.6 — runs.

Added a runtests.jl testset asserting the step is present, gated on 32-bit + Linux, and ordered before setup-julia. Full suite passes; Runic-clean.

Companion changes

• SciMLTesting: folder discovery must skip matrix-only alias sections so GROUP=All / bare Pkg.test don't demand a test/<name>/ folder for the 32-bit lane (separate PR).
• Consumer: SciML/FindFirstFunctions.jl re-adds its Core 32-bit lane once this + the SciMLTesting change are released.

Please ignore until reviewed by @ChrisRackauckas.

🤖 Generated with Claude Code