Am I allowed perform security tests my own servers?

While penetration testing on your own Linode is okay since you've authorized the activity, I would not recommend purposely making your Linode insecure as that could invite unauthorized parties to attempt brute force attacks against your Linode as well.

This could result in your Linode being compromised, possibly resulting in suspicious activity that you will likely need to resolve. If we receive reports of abuse coming from your Linode's IP address as a result of it being compromised and attacking other systems, you will likely see a Terms of Service (ToS) Violation ticket opened on your Linode, which will typically require action and a response from you to resolve. If a ToS ticket does not get resolved, we may apply network restrictions to your Linode to prevent further abuse.

You could prevent unauthorized penetration testing of your Linode by creating a Cloud Firewall that is set to Drop all inbound traffic by default. You could then create a rule within the Cloud Firewall that allows access from the IP address you're using to perform your penetration test. This should limit the network access to your Linode and prevent anyone else from attempting unauthorized access, limiting the likelihood that your Linode will be compromised.

Once your penetration testing is complete, I would recommend deleting the Linode you're testing to ensure that it does not get compromised in the future.