make with_iterations and with_max_len actually usable

[Ekleog-NEAR]

I tried using with_iterations, and came across the issue that bolero::check!() is not actually a RngEngine, hence making it impossible to actually use it from a test.

This fixes it. With these changes, the priority of settings now is:

• Highest-priority, the environment variable (as it’s runtime-configured)
• Lower priority, the .with_iterations() setting (as it’s build-time configured)
• Lowest priority, the 1000/25 default (as it’s hard-coded)

[camshaft]

Nice catch! thanks!