test: fix TestWatcher_SharedParentRefcount on macOS (#25379)

`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

testutil/temp.go

@@ -2,6 +2,7 @@ package testutil
 
 import (
 	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -51,3 +52,22 @@ func CreateTemp(t *testing.T, dir, pattern string) *os.File {
 	})
 	return f
 }
+
+// TempDirResolved returns t.TempDir() with symlinks resolved via
+// filepath.EvalSymlinks. Tests that compare paths against values
+// processed by EvalSymlinks (directly or indirectly) should use
+// this helper so the comparison works on macOS, where the default
+// temp dir lives under /var which is a symlink to /private/var.
+//
+// If EvalSymlinks errors (for example on Windows where the temp
+// path may not resolve cleanly), the raw t.TempDir() result is
+// returned. This matches the lenient behavior already used in
+// existing tests.
+func TempDirResolved(t *testing.T) string {
+	t.Helper()
+	dir := t.TempDir()
+	if resolved, err := filepath.EvalSymlinks(dir); err == nil {
+		return resolved
+	}
+	return dir
+}