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 #2473

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[firelizzard18]

It's also possible to update go.testEnvFile such that it could accept both a string or an array of strings.