ostree-ext: test: fix clippy warning

My clippy is complaining about `len()` being disallowed method. No harm
in changing this.

Signed-off-by: Omer Tuchfeld <omer@tuchfeld.dev>

ostree-ext/src/fixture.rs

@@ -244,7 +244,7 @@ impl SeLabel {
         } else if rootdir == "boot" {
             SeLabel::Boot
         } else if rootdir == "etc" {
-            if p.as_str().len() % 2 == 0 {
+            if p.as_str().as_bytes().len() % 2 == 0 {
                 SeLabel::Etc
             } else {
                 SeLabel::EtcSystemConf