test: enhance SCP plugin with folder ignore and error handling

[appleboy]

• Add TestIgnoreFolder function to test ignoring specific folders during SCP
• Initialize and terminate ssh-agent if SSH_AUTH_SOCK is set
• Lookup user drone-scp and handle potential errors
• Configure Plugin with specific settings for SCP operation
• Execute plugin.Exec() and assert no errors
• Verify that certain files do not exist in the target directory after SCP operation

fix https://github.com/appleboy/drone-scp/issues/190