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

scripts/dev_cli.sh

@@ -47,6 +47,9 @@ CARGO_GIT_REGISTRY_DIR="${CLH_BUILD_DIR}/cargo_git_registry"
 # Full path to the cargo target dir on the host.
 CARGO_TARGET_DIR="${CLH_BUILD_DIR}/cargo_target"
 
+# Let tests know that the special environment is set up.
+RUSTFLAGS="${RUSTFLAGS} --cfg devcli_testenv"
+
 # Send a decorated message to stdout, followed by a new line
 #
 say() {