test: wait for partition device rather than the whole disk

Fixes #32931.

test/units/TEST-58-REPART.sh

@@ -961,7 +961,7 @@ EOF
     # shellcheck disable=SC2064
     trap "rm -rf '$defs' '$imgs' ; losetup -d '$loop'" RETURN ERR
 
-    udevadm wait --timeout 60 --settle "${loop:?}"
+    udevadm wait --timeout 60 --settle "${loop:?}p1" "${loop:?}p2"
 
     # Check that the verity block sizes are as expected
     veritysetup dump "${loop}p2" | grep 'Data block size:' | grep -q '4096'
@@ -1026,7 +1026,7 @@ EOF
     fi
 
     loop=$(losetup -P --show -f "$imgs/zzz")
-    udevadm wait --timeout 60 --settle "${loop:?}"
+    udevadm wait --timeout 60 --settle "${loop:?}p1" "${loop:?}p2"
 
     # Test that /usr/def did not end up in the root partition but other files did.
     mkdir "$imgs/mnt"