fix `cargo run --example server`, missing cert

[yshing]

Running cargo run --example server throws an error caused by missing the pingora/test/keys folder.

This PR copies the cert from the pandora-core folder, so the example runs out of the box.

Hopefully, this will save a few minutes for the next person trying to run cargo run --example server right after cloning.

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[andrewhavck]

This PR copies the cert from the pandora-core folder, so the example runs out of the box.

Mind using a symlink instead?

[yshing]

Hey @andrewhavck updated.

[drcaramelsyrup]

Thanks for contributing! This was merged in 7ce6f4ac1c440756a63b0766f72dbeca25c6fc94.