test: resolve the temp repo path so specs pass on macos

[barrettruth]

Problem

26 specs fail on macOS and pass on Linux: 16 failures and 10 errors across
actions_spec, commands_spec and render_spec.

vim.fn.tempname() returns a path under /var/folders/..., and on macOS /var is
a symlink to /private/var. Git resolves it, the spec does not, so the two disagree
about where the repo is:

tempname()    = /var/folders/.../PF65FX/0
git toplevel  = /private/var/folders/.../PF65FX/0

Confirmed by building the same fixture both ways — with the unresolved path the
rendered diff has no deleted file mode line, with the resolved one it does. Linux
tempname() gives /tmp/... with no symlink, which is why CI never saw it.

Solution

Resolve the path where the fixture is created: vim.fn.resolve(vim.fn.tempname()),
14 sites across 8 spec files. resolve handles the not-yet-created leaf, resolving
the parent symlinks.

Applied to every tempname() in spec/, not only the failing three, so the next
git-backed fixture does not reintroduce it.

802 successes / 0 failures / 0 errors, up from 776/16/10. No change to lua/.