test(AtomicLevel): demonstrate Handler is not vulnerable to XSS (#1477)

Adds a unit test and a fuzz test to demonstrate
that the AtomicLevel handler is not vulnerable to XSS.

I ran the fuzz test on my computer for 10 minutes on my laptop
with no cases where `<...>` was present in the response body.

```
...
fuzz: elapsed: 9m57s, execs: 43145040 (95839/sec), new interesting: 442 (total: 445)
fuzz: elapsed: 10m0s, execs: 43396113 (83690/sec), new interesting: 442 (total: 445)
```

Refs #1476