test: fix TEST-50-DISSECT build on Ubuntu CI

Ubuntu CI's just got the dependencies require dto run this test added,
and it seems the build is different enough from other platforms
that it fails to create the required directories:

cp: cannot create regular file '/var/tmp/systemd-test.JJMOBY/minimal/usr/lib/os-release': No such file or directory

test/TEST-50-DISSECT/test.sh

@@ -37,7 +37,7 @@ test_create_image() {
         )
         oldinitdir=$initdir
         export initdir=$TESTDIR/minimal
-        mkdir -p $initdir
+        mkdir -p $initdir/usr/lib $initdir/etc
         setup_basic_dirs
         install_basic_tools
         cp $os_release $initdir/usr/lib/os-release