Use ioutil package for compatibility with Go 1.13

We specify 1.13 in go.mod, so we should make sure it can be built and
tested on that version. Let's keep using the ioutil package for now,
since it was only deprecated from 1.16.

parser/gotest/json_test.go

@@ -2,6 +2,7 @@ package gotest
 
 import (
 	"io"
+	"io/ioutil"
 	"strings"
 	"testing"
 
@@ -15,7 +16,7 @@ var input = `some other output
 
 func TestJSONReaderReadAll(t *testing.T) {
 	r := newJSONReader(strings.NewReader(input))
-	got, err := io.ReadAll(r)
+	got, err := ioutil.ReadAll(r)
 	if err != nil {
 		t.Fatal(err)
 	}