tests: migrate `libs-through-symlink` to rmake.rs

- Document test intent, backlink to #13890 and fix PR #13903.
- Fix the test logic: the `Makefile` version seems to not actually be
  exercising the "library search traverses symlink" logic, because the
  actual symlinked-to-library is present under the directory tree when
  `bar.rs` is compiled, because the `$(RUSTC)` invocation has an
  implicit `-L $(TMPDIR)`. The symlink itself was actually broken, i.e.
  it should've been `ln -nsf $(TMPDIR)/outdir/$(NAME) $(TMPDIR)` but it
  used `ln -nsf outdir/$(NAME) $(TMPDIR)`.

Co-authored-by: Oneirical <manchot@videotron.ca>

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