Support dynamic counts from checks

Problem

API Security is composed of checks that currently pre-calculate how many tests will be run for a specific parameter being testing. However, this prevents the check from expanding or contracting the number of tests once it has started. This dynamically changing the test count is useful, especially when validating a timing attack is not a false positive.

Example

For timing attacks a sequence like the following should be supported:

1. Test first injection
2. Test second injection
3. Success
   1. Test second injection with small time
   2. Test second injection with larger time
   3. Done!

Proposal

Modify how checks run so that pre-calculated test counts are not used to limit a check's run. Instead checks should run until they signal completion.