Send full git request/response in SSHD tests

Before gitaly!4173 (merged), Gitaly would return success for SSHUploadPack and SSHUploadArchive regardless of the exit code of the git upload-pack|archive process. As a result, the gitlab-sshd acceptance tests could rely on no errors being returned from Gitaly.

Currently these tests send the minimum request needed to start a session, causing the server git process to fail as the 0000 flush packet to end the session is never sent.

This MR fixes the tests by sending the full request/response needed for a successful git operation.