tests: convert one ... expression manually

* tests/ls/stat-vs-dirent: Manually convert `(...)` to $(...),
to avoid automatically producing an invalid result.

tests/ls/stat-vs-dirent

@@ -52,7 +52,7 @@ while :; do
     fi
   fi
 
-  t=`(cd "$t/.."; pwd)`
+  t=$(cd "$t/.."; pwd)
   dev_ino=`stat --format=%d-%i "$t"`
   test $dev_ino = $root_dev_ino && break
 done