json: move _test.v files to vlib/json/tests/ (#22731)

vlib/json/json_encode_with_mut_test.v

@@ -1,42 +0,0 @@
-module main
-
-import json
-
-pub enum PlatformType {
-	unknown
-	osx
-	ubuntu
-	alpine
-}
-
-pub enum CPUType {
-	unknown
-	intel
-	arm
-	intel32
-	arm32
-}
-
-@[heap]
-pub struct Node {
-pub:
-	name string = 'mymachine'
-pub mut:
-	platform    PlatformType
-	cputype     CPUType
-	done        map[string]string
-	environment map[string]string
-}
-
-pub fn (mut node Node) save() ! {
-	data := json.encode(node)
-	dump(data)
-}
-
-fn test_encode_with_mut_struct() {
-	mut n := Node{
-		platform: .osx
-		cputype:  .unknown
-	}
-	n.save() or { panic(err) }
-}