add support for multiple test env files

[jfoshee]

This PR implements support for loading multiple environment files during test execution

New Setting: go.testEnvFiles Added go.testEnvFiles setting that accepts an array of environment file paths Allows loading multiple .env files in a specific order for test execution Later files in the array override variables from earlier files

Backward Compatibility Existing go.testEnvFile setting continues to work as before Marked go.testEnvFile as deprecated with appropriate VS Code deprecation messaging When both settings are used together, go.testEnvFile is processed after all files in go.testEnvFiles

Fixes golang/vscode-go#2473