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

[abhinav]

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

[abhinav]

This change is part of the following stack:

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

_{Change managed by git-spice.}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.45%. Comparing base (0ba452d) to head (3a7b092).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1477   +/-   ##
=======================================
  Coverage   98.45%   98.45%           
=======================================
  Files          53       53           
  Lines        3562     3562           
=======================================
  Hits         3507     3507           
  Misses         46       46           
  Partials        9        9           

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