Fix cargo check --features testing -p stacks-signer

Signed-off-by: Jacinta Ferrant <jacinta.ferrant@gmail.com>

Author: jferrant

stacks-signer/Cargo.toml

@@ -61,4 +61,4 @@ features = ["serde", "recovery"]
 [features]
 default = []
 monitoring_prom = ["prometheus", "tiny_http"]
-testing = []
+testing = ["stacks-common/testing"]