Try to format output JSON

For example tomlc99 outputs its JSON without spaces or newlines; this is
perfectly valid, but failing test cases are hard to read.

README.md

@@ -23,12 +23,9 @@ should run in most environments. It's recommended you use a binary, or a tagged
 release if you build from source especially in CI environments. This prevents
 your tests from breaking on changes to tests in this tool.
 
-To compile from source you will need Go 1.16 or newer (older versions will *not*
-work):
+To compile from source you will need Go 1.18 or newer:
 
-    $ git clone https://github.com/toml-lang/toml-test.git
-    $ cd toml-test
-    $ go build ./cmd/toml-test
+    $ go install github.com/toml-lang/toml-test/cmd/toml-test@latest
 
 This will build a `./toml-test` binary.
 

cmd/toml-test/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"fmt"
 	"io/fs"
 	"os"
@@ -191,6 +192,16 @@ func detailed(r tomltest.Runner, t tomltest.Test) string {
 		b.WriteByte('\n')
 	}
 	showStream(b, "input sent to parser-cmd", t.Input)
+
+	var j map[string]any
+	err := json.Unmarshal([]byte(t.Output), &j)
+	if err == nil {
+		out, err := json.MarshalIndent(j, "", "  ")
+		if err == nil {
+			t.Output = string(out)
+		}
+	}
+
 	if t.OutputFromStderr {
 		showStream(b, "output from parser-cmd (stderr)", t.Output)
 	} else {