Fix forks_should_work_correctly test (#4152)

When we enabled arbitary precision for `serde_json`, the feature leaked
through the whole workspace. The given test fails because of some bug in
serde json when the feature is enabled.

substrate/client/chain-spec/src/extension.rs

@@ -289,7 +289,9 @@ mod tests {
 	fn forks_should_work_correctly() {
 		use super::Extension as _ ;
 
-		let ext: Ext2 = serde_json::from_str(r#"
+		// We first need to deserialize into a `Value` because of the following bug:
+		// https://github.com/serde-rs/json/issues/505
+		let ext_val: serde_json::Value = serde_json::from_str(r#"
 {
 	"test": 11,
 	"forkable": {
@@ -314,6 +316,8 @@ mod tests {
 }
 		"#).unwrap();
 
+		let ext: Ext2 = serde_json::from_value(ext_val).unwrap();
+
 		assert_eq!(ext.get::<Extension1>(), Some(&Extension1 {
 			test: 11
 		}));