test-sysusers: check group creation with matching user entry

test/test-sysusers/test-15.expected-group

@@ -0,0 +1 @@
+root:x:0:

test/test-sysusers/test-15.expected-passwd

@@ -0,0 +1 @@
+root::0:0::/root:/bin/sh

test/test-sysusers/test-15.initial-passwd

@@ -0,0 +1 @@
+root::0:0::/root:/bin/sh

test/test-sysusers/test-15.input

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# Create proper group when matching user entry is in place.
+#
+#Type Name ID GECOS        HOMEDIR
+u     root 0  "Super User" /root