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/run_integration_tests_aarch64.sh

@@ -191,7 +191,9 @@ if [ $RES -ne 0 ]; then
     exit 1
 fi
 
+# Common configuration for every test run
 export RUST_BACKTRACE=1
+export RUSTFLAGS="$RUSTFLAGS"
 
 cargo build --features mshv --all --release --target "$BUILD_TARGET"