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_live_migration.sh

@@ -83,7 +83,10 @@ PAGE_NUM=$((12288 * 1024 / HUGEPAGESIZE))
 echo "$PAGE_NUM" | sudo tee /proc/sys/vm/nr_hugepages
 sudo chmod a+rwX /dev/hugepages
 
+# Common configuration for every test run
 export RUST_BACKTRACE=1
+export RUSTFLAGS="$RUSTFLAGS"
+
 time cargo nextest run $test_features --retries 3 --no-fail-fast --test-threads=$(($(nproc) / 4)) "live_migration_parallel::$test_filter" -- ${test_binary_args[*]}
 
 RES=$?