Add test from PostReceivePack to hooks
This MR improves the hooks tests in the following ways:
- runs the hooks from the test repo's directory
- adds test to go from PostReceivePack all the way through hooks
- adds test to go from SSHReceivePack all the way through hooks
- refactors test code into
testhelper
package for the sake of code sharing
Edited by GitLab Release Tools Bot