Add flexible source-free load remap regression test

[jack-champagne]

What this locks in

Adds test/test_flexible_load.jl: an end-to-end regression for the flexible
source-free load that remap! + load_package_image exist to enable —
loading a precompiled package image whose dependency has since been rebuilt
under a new build-id, by rewriting the recorded build-id so the image
relinks against the new dependency at load time, with no source.

The test builds its own two trivial packages (User -> Dep) in an isolated
temp depot (never touches ~/.julia), precompiles Dep@v1 + User, stashes
the images, then perturbs Dep and recompiles to Dep@v2. It then runs four
scenarios, each in a separate julia child process (two builds of the same
module name cannot coexist in one process):

• A — positive control: load User against the Dep it was built with →
  User.hello() == "User sees: Dep v1".
• B1 — no remap: load the stale User against Dep@v2 → clean failure in
  resolve_dep (Cannot resolve dependency: module 'Dep' ... not found), no
  crash, no spurious load.
• B2 — the regression: remap User's recorded Dep build-id to Dep@v2
  in both the .so (read by the C restore step) and the .ji header (read
  by resolve_dep), then load source-free → loads and relinks:
  User.hello() == "User sees: Dep v2".
• B3 — negative control: remap to a bogus build-id → clean failure (no
  crash, no spurious load).

Notes

• Self-contained: builds its own fixtures, does not depend on an external
  bundle or JSD_FIXTURE_DEPOT.
• Skips cleanly via @test_skip when a split .so package image cannot be
  produced (no C toolchain, or the build otherwise fails) instead of failing.
• Child processes run with an isolated JULIA_DEPOT_PATH and a stdlib-only
  load path; the user depot is never written.
• Does not modify any existing test file or src/.

Result

Full suite green locally on Julia 1.12.6 (Pkg.test()): 198 Pass, 0 Fail,
with the round-trip actually exercised (not skipped).

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