tests: add integration test for exec()

[cognifloyd]

Part of go-vela/community#585

This adds some integration tests for the exec() function. It only adds them for the docker runtime, as it is currently hanging for kubernetes. I will fix the kubernetes runtime in follow-up PRs.

I also had to add a mock API server for the worker. I based it on the server's mock.

[codecov]

Codecov Report

Merging #336 (ee0c132) into master (6c8171c) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   81.17%   81.04%   -0.14%     
==========================================
  Files          68       70       +2     
  Lines        5274     5417     +143     
==========================================
+ Hits         4281     4390     +109     
- Misses        849      873      +24     
- Partials      144      154      +10     

Impacted Files	Coverage Δ
cmd/vela-worker/exec.go	74.16% <100.00%> (ø)
runtime/setup.go	100.00% <100.00%> (ø)
cmd/vela-worker/client.go	76.92% <0.00%> (ø)