libjob/test: add informational test for string_unwrap()

Problem: When there is a local problem configuring flux-security,
diagnosis from the unwrap unit tests is difficult because errors are
suppressed in the tests.

Add a test that captures any error from sign_unwrap() for help
debugging future issues.

Author: grondo

src/common/libjob/test/unwrap.c

@@ -47,6 +47,16 @@ static void test_api (unwrap_f unwrap)
         "error.text says: %s",
         error.text);
 
+    userid = 0;
+    result = (*unwrap) (s, false, NULL, &error);
+    ok (result != NULL && userid == 0,
+        "unwrap_string() works with NULL userid");
+    if (result == NULL)
+        diag ("got error: %s", error.text);
+    is (result, "bar",
+        "got expected result");
+    free (result);
+
     userid = 0;
     result = (*unwrap) (s, false, NULL, NULL);
     ok (result != NULL && userid == 0,