chore(invariant): config defaults, add max test threads config

[grandizzy]

Motivation

Closes #7848 Closes #6513 - see https://github.com/foundry-rs/foundry/issues/6513#issuecomment-2110172882

Solution

• change default value for invariant depth to 500
• change default shrink run limit to 5000
• add max_test_threads config to specify number of threads for running tests. If not specified it defaults to rayon determined number

(will follow up with PR to update book when this is merged)