This PR resolves the issues that are run into when running two separate unit tests in the same crate (see issue #58). Previously, doing this would cause the test runners to use the same crate directory, leading to one overwriting the other's manifest. This resulted in confusing errors and tests failing to no fault of the user.

To solve this, separate directories are defined for each run, using a hash of the test paths used by the TestCase. Therefore, the following now works correctly:

use trybuild::TestCases;

#[test]
fn test_1() {
    TestCases::new().compile_fail("compile_failures/1.rs");
}

#[test]
fn test_2() {
    TestCases::new().compile_fail("compile_failures/2.rs");
}

This does add a dependency on rustc-hash, the hasher used within rustc. I would say this is a trustworthy additional dependency, as it is owned by the Rust core team. The hash is not cryptographic, but it doesn't need to be for this use case. If you would rather not add this dependency, let me know and we can discuss another way.