test(system-defaults): switch to single top-level require so undercover instruments

The helper-functions test was per-test reloading system-defaults.el
via `(load ...)' inside a `cl-letf' sandbox that stubs the
side-effecting primitives (server-start, set-locale-environment,
etc).  Tests passed, but the coverage gauge stayed stuck at 1/12
because undercover.el only instruments the first load of a matching
source; subsequent re-loads inside test bodies don't get tracked,
so the function bodies showed as uncovered even though every test
called them.

Rewrite the test to call `(require 'system-defaults)' once at top
level, wrapped in the same `cl-letf' stubs.  The functions get
instrumented exactly once.  Drop the now-unused per-test sandbox
macro.  Add two more tests for the `(when (memq ...))' list-without-
comp guard and the non-string-message format branch so coverage
reaches 12/12.

(`test-system-defaults-vc-follow-symlinks.el' still uses the
per-test `(load ...)' pattern because that test *is* the
load-side-effect verification, not a function-body test.)

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