Add integration test: ALLOW_EMPTY_CHALLENGE_SOLUTION

[joshlf]

This is part of the larger effort in #53.

Behaviors to test:

• When ALLOW_EMPTY_CHALLENGE_SOLUTION is not set, challenge solutions are enforced
• When ALLOW_EMPTY_CHALLENGE_SOLUTION is not set:
  • When the provided challenge solution is empty, the request is allowed to continue
  • When the provided challenge solution is non-empty, the solution is enforced