json2: decode fn returns ?T; add new tests (#6933)

vlib/x/json2/decoder_test.v

@@ -0,0 +1,61 @@
+import x.json2
+
+fn test_raw_decode_string() {
+	str := json2.raw_decode('"Hello!"') or {
+		assert false
+		json2.Any{}
+	}
+	assert str.str() == 'Hello!'
+}
+
+fn test_raw_decode_number() {
+	num := json2.raw_decode('123') or {
+		assert false
+		json2.Any{}
+	}
+	assert num.int() == 123
+}
+
+fn test_raw_decode_array() {
+	raw_arr := json2.raw_decode('["Foo", 1]') or {
+		assert false
+		json2.Any{}
+	}
+	arr := raw_arr.arr()
+	assert arr[0].str() == 'Foo'
+	assert arr[1].int() == 1
+}
+
+fn test_raw_decode_bool() {
+	bol := json2.raw_decode('false') or {
+		assert false
+		json2.Any{}
+	}
+	assert bol.bool() == false
+}
+
+fn test_raw_decode_map() {
+	raw_mp := json2.raw_decode('{"name":"Bob","age":20}') or {
+		assert false
+		json2.Any{}
+	}
+	mp := raw_mp.as_map()
+	assert mp['name'].str() == 'Bob'
+	assert mp['age'].int() == 20
+}
+
+fn test_raw_decode_null() {
+	nul := json2.raw_decode('null') or {
+		assert false
+		json2.Any{}
+	}
+	assert nul is json2.Null
+}
+
+fn test_raw_decode_invalid() {
+	json2.raw_decode('1z') or {
+		assert err == '[json] invalid JSON. (0:0)'
+		return
+	}
+	assert false
+}