fix(test): make install_docker_engine os-release detection mockable so setup-linux.bats passes on macOS

[saadqbal]

Closes#232.

Problem

Two bats tests in scripts/tests/setup-linux.bats failed on macOS dev hosts (green in CI on Linux):

• install_docker_engine: Amazon Linux -> dnf docker
• install_docker_engine: RHEL clone (#719) -> docker-ce dnf repo

install_docker_engine() gated the Amazon Linux and AlmaLinux/Rocky branches on [[ -f /etc/os-release ]] && grep -qi ... /etc/os-release. The suite mocks grep, but a bash builtin file-test ([[ -f /etc/os-release ]]) can't be intercepted by a function mock. On macOS /etc/os-release doesn't exist, so both branches short-circuited and the function fell through to the get.docker.com path — the asserted strings were never recorded. The pacman/zypper/ubuntu branches passed because they gate on has <cmd> (mockable via PRESENT_CMDS).

CI was unaffected: unit-bash runs on ubuntu (os-release present), and the real RHEL/Amazon docker paths are independently exercised by the distro-prereqs matrix in actual almalinux/amazonlinux containers.

Fix

• scripts/lib/setup-linux.sh — install_docker_engine reads the os-release path from ${TB_OS_RELEASE_FILE:-/etc/os-release}, used for both the [[ -f ]] test and the grep. Defaults to the exact prior behaviour when the env var is unset (production / CI).
• scripts/tests/setup-linux.bats — setup() writes a real os-release fixture per $TEST_DISTRO to a temp file and exports TB_OS_RELEASE_FILE. This replaces the old grep mock, so distro detection (real [[ -f ]] + real grep) is exercised on every dev host — keeping the coverage meaningful on macOS rather than skipping it.

Verification

• bats scripts/tests/setup-linux.bats → 20/20 pass on macOS (was 18/20).
• Verified on both this host's grep (ugrep 7.5.0) and stock /usr/bin/grep (BSD grep 2.6.0-FreeBSD, GNU-compatible) — the \| BRE and ERE alternations both match, so real grep is robust across macOS dev hosts.
• No behaviour change when TB_OS_RELEASE_FILE is unset.
• No new shellcheck findings (additions are quoted; remaining SC2086/SC2016 are pre-existing and untouched).

🤖 Generated with Claude Code

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Note

Low Risk
Test-only hook with default path unchanged; no change to installer behavior when TB_OS_RELEASE_FILE is unset.

Overview
Fixes macOS failures in setup-linux.bats for Amazon Linux and AlmaLinux Docker install paths by making os-release detection testable without mocking bash [[ -f ]].

install_docker_engine now reads the os-release path from TB_OS_RELEASE_FILE (default /etc/os-release), used for both the file existence check and grep distro matching. Unset env var keeps production/CI behavior unchanged.

The bats setup() drops the grep mock and adds write_os_release, which writes a temp fixture per TEST_DISTRO and exports TB_OS_RELEASE_FILE. Amazon and RHEL-clone tests call write_os_release so real [[ -f ]] + grep exercise the intended branches on hosts without /etc/os-release.

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