fuzz: add a fuzzer for user records

Add a simple fuzzer that verifies our machinery for parsing user records
from JSON works as intended.

The initial corpus was created with the help of Claude, so we have a
bunch of valid user records with as much fields as possible for the
initial corpus.

test/fuzz/fuzz-user-record/basic-regular.json

@@ -0,0 +1,9 @@
+{
+    "userName": "testuser",
+    "realName": "Test User",
+    "uid": 1000,
+    "gid": 1000,
+    "homeDirectory": "/home/testuser",
+    "shell": "/bin/bash",
+    "disposition": "regular"
+}