Derivation::to_json_string and test

Use `serde_json` at test time.

Cargo.lock

@@ -156,6 +156,12 @@ dependencies = [
  "either",
 ]
 
+[[package]]
+name = "itoa"
+version = "1.0.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "92ecc6618181def0457392ccd0ee51198e065e016d1d527a7ac1b6dc7c1f09d2"
+
 [[package]]
 name = "lazy_static"
 version = "1.5.0"
@@ -313,6 +319,7 @@ dependencies = [
  "nix-bindings-util",
  "nix-bindings-util-sys",
  "pkg-config",
+ "serde_json",
  "tempfile",
  "zerocopy",
 ]
@@ -461,6 +468,48 @@ dependencies = [
  "windows-sys 0.61.1",
 ]
 
+[[package]]
+name = "serde"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9a8e94ea7f378bd32cbbd37198a4a91436180c5bb472411e48b5ec2e2124ae9e"
+dependencies = [
+ "serde_core",
+]
+
+[[package]]
+name = "serde_core"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41d385c7d4ca58e59fc732af25c3983b67ac852c1a25000afe1175de458b67ad"
+dependencies = [
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_derive"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d540f220d3187173da220f885ab66608367b6574e925011a9353e4badda91d79"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "serde_json"
+version = "1.0.149"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "83fc039473c5595ace860d8c4fafa220ff474b3fc6bfdb4293327f1a37e94d86"
+dependencies = [
+ "itoa",
+ "memchr",
+ "serde",
+ "serde_core",
+ "zmij",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"
@@ -640,3 +689,9 @@ dependencies = [
  "quote",
  "syn",
 ]
+
+[[package]]
+name = "zmij"
+version = "1.0.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dfcd145825aace48cff44a8844de64bf75feec3080e0aa5cdbde72961ae51a65"