fix: add missing testdata

Commit 41edee2 forgot to add golang.org/x/sys/unix stub. Whoops.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

errorlint/testdata/src/golang.org/x/sys/unix/stubs.go

@@ -0,0 +1,16 @@
+package unix
+
+import "syscall"
+
+const (
+	EPERM  = syscall.Errno(0x1)
+	ENOENT = syscall.Errno(0x2)
+)
+
+func Rmdir(string) error {
+	return ENOENT
+}
+
+func Kill(int, syscall.Signal) error {
+	return EPERM
+}