Make EnvTestConfig internals private and add functions

[leighmcculloch]

What problem does your feature solve?

The EnvTestConfig type contains feature flags that can be enabled/disabled to control how the test Env behaves. Because the type has a list of pub fields it's not possible to add new fields to the config between major versions, because that would break pattern matching of the type, and value creation that requires all fields to be specified.

What would you like to see?

Make EnvTestConfig internals private and add functions.

Suggest the following interface:

Before

let config = EnvTestConfig { capture_snapshot_at_drop: false };
let env = Env::new_with_config(config);

After

let config = EnvTestConfig::default()
    .set_capture_snapshot_at_drop(false);
let env = Env::new_with_config(config);