Fix `cargo check` for `pallet-contracts-proc-macro` (#12706)

* fix `cargo check` for pallet-contracts-proc-macro

* add test for cargo-check of pallet-contracts-proc-macro

* remove cargo-check-contracts-proc-macro

https://github.com/paritytech/substrate/pull/12706/files#r1022783937

substrate/frame/contracts/proc-macro/Cargo.toml

@@ -17,7 +17,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1"
 quote = "1"
-syn = "1.0.98"
+syn = { version = "1.0.98", features = ["full"] }
 
 [dev-dependencies]