hclsyntax: use go-test/deep for comparing parse test results

We were previously using an ugly combination of "pretty" and "spew" to
do this, which never really quite worked because of limitations in each
of those.

deep.Equal doesn't produce quite as much detailed information as the
others, but it has the advantage of showing exactly where a difference
exists rather than forcing us to hunt through a noisy diff to find it.

hcl/hclsyntax/parser_test.go

@@ -1,12 +1,10 @@
 package hclsyntax
 
 import (
-	"reflect"
 	"testing"
 
-	"github.com/davecgh/go-spew/spew"
+	"github.com/go-test/deep"
 	"github.com/hashicorp/hcl2/hcl"
-	"github.com/kylelemons/godebug/pretty"
 	"github.com/zclconf/go-cty/cty"
 )
 
@@ -1445,12 +1443,6 @@ block "valid" {}
 		},
 	}
 
-	prettyConfig := &pretty.Config{
-		Diffable:          true,
-		IncludeUnexported: true,
-		PrintStringers:    true,
-	}
-
 	for _, test := range tests {
 		t.Run(test.input, func(t *testing.T) {
 			file, diags := ParseConfig([]byte(test.input), "", hcl.Pos{Byte: 0, Line: 1, Column: 1})
@@ -1463,12 +1455,9 @@ block "valid" {}
 
 			got := file.Body
 
-			if !reflect.DeepEqual(got, test.want) {
-				diff := prettyConfig.Compare(test.want, got)
-				if diff != "" {
-					t.Errorf("wrong result\ninput: %s\ndiff:  %s", test.input, diff)
-				} else {
-					t.Errorf("wrong result\ninput: %s\ngot:   %s\nwant:  %s", test.input, spew.Sdump(got), spew.Sdump(test.want))
+			if diff := deep.Equal(got, test.want); diff != nil {
+				for _, problem := range diff {
+					t.Errorf(problem)
 				}
 			}
 		})