Make sure you've also imported the private key you would've gotten while creating the project in sandbox. And enter the correct keystore passwords in the APIClient Test class.
If it still doesn't work, you can delete the project from sandbox(which will make the old credentials invalid) and recreate the same project with new credentials and try to test the API again.
We still think this is a certificate issue. Can you please print out the keytool output of your key store? There should be three certificates inside of it. Please share with us your findings of the output.
Can you please let us know if the issue is resolved or if you need help with anything else?
Thanks for the update. Have a wonderful week.