Adds integration test based on adder collator (#1928)

* Adds integration test based on adder collator

This adds an integration test for parachains that uses the adder
collator. The test will start two relay chain nodes and one collator and
waits until 4 blocks are build and enacted by the parachain.

* Make sure the integration test is run in CI

* Fix wasm compilation

* Update parachain/test-parachains/adder/collator/src/lib.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Update cli/src/command.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

polkadot/scripts/gitlab/test_linux_stable.sh

@@ -3,4 +3,4 @@
 #shellcheck source=lib.sh
 source "$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )/lib.sh"
 
-time cargo test --all --release --verbose --locked --features runtime-benchmarks
+time cargo test --all --release --verbose --locked --features=runtime-benchmarks --features=real-overseer