`TestWatcher_SharedParentRefcount` was deterministically broken on
macOS: `t.TempDir()` lives under `/var` which is a symlink to
`/private/var`, but the watcher canonicalizes paths via
`filepath.EvalSymlinks` before storing them, so the test's `w.dirs[dir]`
lookup missed and returned `0` instead of `2`.
Adds `testutil.TempDirResolved`, a shared helper that returns
`t.TempDir()` with symlinks resolved and falls back to the raw temp dir
on error (Windows-friendly). Migrates the matching inline
`EvalSymlinks(t.TempDir())` callsites in
`agent/agentgit/agentgit_test.go` to use it.
Closes https://github.com/coder/internal/issues/1531
Ethan