Fix golangci-lint for context string key unittest

[zhouzhuojie]

Description

Motivation and Context

Fix golangci-lint:

#!/bin/bash -eo pipefail
make ci
Running verify_lint
pkg/handler/subject_test.go:26:31: SA1029: should not use built-in type string as key for value; define your own type to avoid collisions (staticcheck)
	ctx = context.WithValue(ctx, interface{}(config.Config.JWTAuthUserProperty), &jwt.Token{})
	                             ^
pkg/handler/subject_test.go:29:31: SA1029: should not use built-in type string as key for value; define your own type to avoid collisions (staticcheck)
	ctx = context.WithValue(ctx, interface{}(config.Config.JWTAuthUserProperty), &jwt.Token{
	                             ^
make: *** [Makefile:66: verify_lint] Error 1

Exited with code exit status 2

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.