- 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>
