test: make busybox TEST-13-only dependency

to make running other tests a bit easier on distributions which don't have
busybox in repositories (CentOS).

test/TEST-13-NSPAWN-SMOKE/test.sh

@@ -11,6 +11,7 @@ TEST_NO_NSPAWN=1
 test_append_files() {
     (
         local workspace="${1:?}"
+        dracut_install busybox
 
         if selinuxenabled >/dev/null; then
             dracut_install selinuxenabled

test/test-functions

@@ -126,7 +126,6 @@ BASICTOOLS=(
     base64
     basename
     bash
-    busybox
     capsh
     cat
     chmod