TEST-13-NSPAWN: make sure we don't load libnss_systemd

Let's make sure we don't load libnss_systemd.so from bash as the
necessary environment variables aren't set to make that work when
we're running with sanitizers enabled.

We can't add a sanitizer wrapper for bash as the wrapper runs using
bash so you end up in a loop.

test/units/TEST-13-NSPAWN.nspawn.sh

@@ -179,6 +179,10 @@ elif [[ $1 == initgroups ]]; then
 fi
 EOF
     chmod +x "$root/bin/getent"
+    # The useradd is important here so the user is added to /etc/passwd. If the user is not in /etc/passwd,
+    # bash will end up loading libnss_systemd.so which breaks when libnss_systemd.so is built with sanitizers
+    # as bash isn't invoked with the necessary environment variables for that.
+    useradd --root="$root" --uid 1000 --user-group --create-home testuser
     systemd-nspawn --directory="$root" bash -xec '[[ $USER == root ]]'
     systemd-nspawn --directory="$root" --user=testuser bash -xec '[[ $USER == testuser ]]'
 
@@ -672,8 +676,10 @@ fi
 EOF
     chmod +x "$root/bin/getent"
 
-    mkdir -p "$root/home/testuser"
-    chown 1010:1010 "$root/home/testuser"
+    # The useradd is important here so the user is added to /etc/passwd. If the user is not in /etc/passwd,
+    # bash will end up loading libnss_systemd.so which breaks when libnss_systemd.so is built with sanitizers
+    # as bash isn't invoked with the necessary environment variables for that.
+    useradd --root="$root" --uid 1010 --user-group --create-home testuser
 
     cmd='PERMISSIONS=$(stat -c "%u:%g" /home/testuser/file); if [[ $PERMISSIONS != "1010:1010" ]]; then echo "*** wrong permissions: $PERMISSIONS"; return 1; fi; touch /home/testuser/other_file'
     if ! SYSTEMD_LOG_TARGET=console \