test: use mkdir -p in TEST-25-IMPORT

[   15.896174] TEST-25-IMPORT.sh[473]: + mkdir /var/tmp/scratch
[   15.902524] TEST-25-IMPORT.sh[519]: mkdir: cannot create directory ‘/var/tmp/scratch’: File exists

https://github.com/systemd/systemd/actions/runs/12248114409/job/34167155679?pr=35520

test/units/TEST-25-IMPORT.sh

@@ -64,10 +64,10 @@ test ! -f /var/lib/machines/testimage4.raw
 # → And now, let's test directory trees ← #
 
 # Set up a directory we can import
-mkdir /var/tmp/scratch
+mkdir -p /var/tmp/scratch
 mv /var/tmp/testimage.raw /var/tmp/scratch/
 touch /var/tmp/scratch/anotherfile
-mkdir /var/tmp/scratch/adirectory
+mkdir -p /var/tmp/scratch/adirectory
 echo "piep" >/var/tmp/scratch/adirectory/athirdfile
 
 # Test import-fs
@@ -78,7 +78,7 @@ machinectl image-status scratch
 # Test export-tar
 machinectl export-tar scratch /var/tmp/scratch.tar.gz
 test -f /var/tmp/scratch.tar.gz
-mkdir /var/tmp/extract
+mkdir -p /var/tmp/extract
 (cd /var/tmp/extract ; tar xzf /var/tmp/scratch.tar.gz)
 diff -r /var/tmp/scratch/ /var/tmp/extract/
 rm -rf /var/tmp/extract