refactor(tests): reorganize test files to match source modules

Restructure 19 test files into 11 (10 module tests + 1 helpers) so each
test file corresponds to its source module.  Remove 436 low-value tests
(getter/setter round-trips, defcustom defaults, feature-provided checks,
repeated validation patterns) keeping 1,342 intent-communicating tests.

New files: elinit-test-log.el, elinit-test-overrides.el,
elinit-test-libexec.el.

Dissolved: elinit-test-dag.el, elinit-test-plan.el,
elinit-test-restart.el, elinit-test-keywords.el,
elinit-test-validation.el, elinit-test-identity.el,
elinit-test-logformat.el, elinit-test-logging.el,
elinit-test-policy.el, elinit-test-targets.el, elinit-test-rlimits.el.

Also fix two melpazoid lint issues: use (temporary-file-directory) in
sandbox X11 socket path, bare defvar for tabulated-list-header-string.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs: add URLs for recommended systemd-free distros

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>