Restructure integration tests

[ludfjig]

We currently have 2 different integration test files: integration_test.rs and sandbox_host_test.rs. The difference between these is unclear.

Certain integration tests are tested against both two types of guests (c and rust) by using new_uninit helper function, which will return either rust/c guests depending on an environment variable. We should consolidate all integration tests that need to run against both rust and c guests into its own separate file. Additionally, we should not depend on environment variables for testing, and instead using something like this (just an example)

#[test]
fn some_test() {
    with_c_or_rust_sandbox(|sandbox: MultiUseSandbox| {
        /// test logic here
    });
}

Where with_c_or_rust_sandbox will run twice, one with c guest and one with rust guests.

The rest of the integration tests (that only exercises rust or only exercises c) can live in a separate files.

In addition, running just test will right now run the same WIT tests twice, because it's ran with both GUEST=c and GUEST=rust, even though it of course only uses the rust guests. This is slow and unnecessary.