proptest_path in attr macro

[cameron1024]

Adds an argument to the attribute macro proptest_path = "some::path::to::proptest", which changes where the macro assumes it can find the proptest crate. I.e., if you provide proptest_path = "some::path::to::proptest, the macro will emit:

impl some::path::to::proptest::prelude::Arbitrary for ... {}

Motivation

At Zed, we have a custom test macro that injects some special arguments, and provides a limited form of property testing. I am improving this macro by using proptest instead of uniformly sampling from a distribution, but the current macro assumes that ::proptest will always be accessible. Adding this parameter allows the gpui::test macro to expand to #[gpui::proptest::property_test(proptest_path = "gpui::proptest") fn ..., which removes the need to add proptest as a dependency in every crate that wants to write a test.