tests: enable cargo test --workspace + #[cfg(devcli_testenv)]

TL;DR: Massive quality of life improvement for devs

Cloud Hypervisor uses the Cargo test framework for multiple tests:

- normal unit tests
- unit tests requiring special environment (the Tap device tests)
- integration tests requiring a special environment

This prevented the execution of `cargo test --workspace`, which results
in a very poor developer experience. Although
`./scripts/run_unit_tests.sh` exists, there are valid reasons why devs
cannot or even don't want to use it.

By adding a new `chv_testenv` rustc config, we can conditionally only
activate tests when the `./scripts/` magic runs them. This improves
the general developer experience by a lot.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

tests/integration.rs

@@ -2,6 +2,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 //
+#![cfg(devcli_testenv)]
 #![allow(clippy::undocumented_unsafe_blocks)]
 // When enabling the `mshv` feature, we skip quite some tests and
 // hence have known dead-code. This annotation silences dead-code