When automating a test on iOS, there's an extra dimension to reproducing a command: the keychain. The keychain is involved in signing your test so it can get onto the actual iOS device. If you get an error like:
Testing failed:
No signing certificate "iOS Development" found: No "iOS Development" signing certificate matching team ID "BLA5BLA555" with a private key was found.
Code signing is required for product type 'Application' in SDK 'iOS 10.2'
Then it might be the case that your keychain is for some reason unavailable to your buildslave, despite using the same user and environment. To check, you can run
security find-identity -p codesigning -v
both from within the buildslave and as a human. If the output is different, something's up. Copying your iPhone Developer certificate to the System keychain using the Keychain Access utility application may solve this.