fix (zebra-chain): make test and bench targets build using 2021 edition (#3404)

The 2021 edition activates the v2 cargo resolver.

This broke implicit feature resolution,
but only when `zebra-chain`'s dev or bench targets were compiled by themselves.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

zebra-chain/Cargo.toml

@@ -81,11 +81,14 @@ proptest-derive = "0.3"
 rand = "0.8"
 rand_chacha = "0.3"
 
+tokio = "1.15.0"
+
 zebra-test = { path = "../zebra-test/" }
 
 [[bench]]
 name = "block"
 harness = false
+required-features = ["bench"]
 
 [[bench]]
 name = "redpallas"