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_x86_64.sh

@@ -177,14 +177,16 @@ ulimit -l unlimited
 # Set number of open descriptors high enough for VFIO tests to run
 ulimit -n 4096
 
+# Common configuration for every test run
 export RUST_BACKTRACE=1
+export RUSTFLAGS="$RUSTFLAGS"
+
 time cargo nextest run $test_features --retries 3 --no-fail-fast --test-threads=$(($(nproc) / 4)) "common_parallel::$test_filter" -- ${test_binary_args[*]}
 RES=$?
 
 # Run some tests in sequence since the result could be affected by other tests
 # running in parallel.
 if [ $RES -eq 0 ]; then
-    export RUST_BACKTRACE=1
     cargo nextest run $test_features --retries 3 --no-fail-fast --test-threads=1 "common_sequential::$test_filter" -- ${test_binary_args[*]}
     RES=$?
 fi
@@ -192,7 +194,6 @@ fi
 # Run tests on dbus_api
 if [ $RES -eq 0 ]; then
     cargo build --features "mshv,dbus_api" --all --release --target "$BUILD_TARGET"
-    export RUST_BACKTRACE=1
     # integration tests now do not reply on build feature "dbus_api"
     time cargo nextest run $test_features --retries 3 --no-fail-fast --test-threads=$(($(nproc) / 4)) "dbus_api::$test_filter" -- ${test_binary_args[*]}
     RES=$?
@@ -201,14 +202,12 @@ fi
 # Run tests on fw_cfg
 if [ $RES -eq 0 ]; then
     cargo build --features "mshv,fw_cfg" --all --release --target "$BUILD_TARGET"
-    export RUST_BACKTRACE=1
     time cargo nextest run $test_features --no-tests=warn --retries 3 --no-fail-fast --test-threads=$(($(nproc) / 4)) "fw_cfg::$test_filter" -- ${test_binary_args[*]}
     RES=$?
 fi
 
 if [ $RES -eq 0 ]; then
     cargo build --features "mshv,ivshmem" --all --release --target "$BUILD_TARGET"
-    export RUST_BACKTRACE=1
     time cargo nextest run $test_features --retries 3 --no-fail-fast --test-threads=$(($(nproc) / 4)) "ivshmem::$test_filter" -- ${test_binary_args[*]}
     RES=$?
 fi