withGitCrypt step unit test

[dtschan]

Implement unit test for withGitCrypt step which tests:

• Works whether git-crypt is in PATH or not
  • Installs git-crypt if not in PATH
• If credentialsId is not null
  • Calls git-crypt unlock ${gitCryptFile} where gitCryptFile is the git-crypt key stored in the Jenkins credential identified by credentialsId
• Calls the passed closure
• If credentialsId is not null
  • Calls git-crypt lock, even when the passed closure threw an exception