allow to override explicit proptest_config from env vars

[tzemanovic]

When the explicit #[proptest_config(Config ...)] is being used in a test, for any fields that are not Config::default() the PROPTEST_ env vars are ignored. I think it would be better to allow the env vars to override the config as it enables ad hoc changes without recompiling, but it is potentially a breaking change if someone relies on the current behavior.

The fix should be easy to do using fn contextualize_config from mod config.