[enh] helpers: add some tests for system user create that were lying in my uncommited files ?

tests/test_helpers.d/ynhtest_user.sh

@@ -0,0 +1,25 @@
+
+ynhtest_system_user_create() {
+    username=$(head -c 12 /dev/urandom | md5sum | head -c 12)
+    
+    ! ynh_system_user_exists --username="$username"
+    
+    ynh_system_user_create --username="$username"
+    
+    ynh_system_user_exists --username="$username"
+
+    ynh_system_user_delete --username="$username"
+
+    ! ynh_system_user_exists --username="$username"
+}
+
+ynhtest_system_user_with_group() {
+    username=$(head -c 12 /dev/urandom | md5sum | head -c 12)
+    
+    ynh_system_user_create --username="$username" --groups="ssl-cert,ssh.app"
+    
+    grep -q "^ssl-cert:.*$username" /etc/group
+    grep -q "^ssh.app:.*$username" /etc/group
+
+    ynh_system_user_delete --username="$username"
+}