add copyright headers and one comment

Author: Abhijeet V

.golangci.yaml

@@ -1,3 +1,6 @@
+# Copyright (c) HashiCorp, Inc.
+# SPDX-License-Identifier: MPL-2.0
+
 version: "2"
 issues:
   max-issues-per-linter: 0 # show all issues found by each linter

hcltest/mock_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/zclconf/go-cty/cty"
 )
 
+// Assert mocks are compatible with the interfaces they implement.
 var _ hcl.Body = mockBody{}
 var _ hcl.Expression = mockExprLiteral{}
 var _ hcl.Expression = mockExprVariable("")