add integration tests

[GregoryDosh]

Adds integration tests for the SCP & SSH plugins

Example of a forced failure with a bad password in the compose file: https://github.com/go-vela/vela-openssh/runs/4885551347?check_suite_focus=true
Fixed run that runs successfully: https://github.com/go-vela/vela-openssh/runs/4885632012?check_suite_focus=true

[codecov]

Codecov Report

Merging #2 (bd23676) into ready-for-pr (c6dfdfb) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           ready-for-pr       #2   +/-   ##
=============================================
  Coverage         93.40%   93.40%           
=============================================
  Files                 3        3           
  Lines               288      288           
=============================================
  Hits                269      269           
  Misses               13       13           
  Partials              6        6