tests: add CI user creation to simulate cloud VM environments

Create a 'ci' user to match upstream behaviour. This simulates cloud VM
environments where ID 1000 is already taken. This ensures that test
users start the ID assignment at 1001, making container tests consistent
with cloud CI environments.

Link: <https://github.com/shadow-maint/shadow/pull/1615>
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

main.fmf

@@ -41,6 +41,11 @@
               - podman exec builder dnf update -y
               - podman exec builder dnf install -y util-linux expect
 
+        - name: Create CI user to simulate cloud VM environment
+          how: shell
+          script:
+              - podman exec builder useradd ci
+
     discover:
         how: fmf