damus : test: pass keypair instead of privkey for test cases

Tests were not building due to recent changes in the Damus source code that replaced privkey with keypair. This patch extends those changes to the test cases, allowing the tests to build and pass.

Signed-off-by: Jon Marrs <[email protected]>
Signed-off-by: William Casarin <[email protected]>
https://github.com/damus-io/damus/commit/dd29e871465332828ddeed59b151177166f7601e