Add exec test step plugin

[mimir-d]

Description

The exec plugin runs a given binary or executable and determines whether the test passed based on criteria specified by options. The executables are run using a transport that abstracts the mechanism of launching the process.
More info in readme.md in plugin folder.

Testing

unit tests

> go test ./plugins/teststeps/exec
ok      github.com/facebookincubator/contest/plugins/teststeps/exec     0.003s

integration tests

• build container

  # docker build -f ./docker/contest/Dockerfile .

• get into container

  # docker run -it <image> /bin/bash

• run the tests

  root@9e6dabe09b08:/go/src/github.com/facebookincubator/contest# service ssh restart
  [ ok ] Restarting OpenBSD Secure Shell server: sshd.
  root@9e6dabe09b08:/go/src/github.com/facebookincubator/contest# go build ./cmds/exec_agent/
  root@9e6dabe09b08:/go/src/github.com/facebookincubator/contest# go test -race -tags=integration,unsafe ./tests/integ/plugins/ -count=1
  ok      github.com/facebookincubator/contest/tests/integ/plugins        3.953s

[codecov[bot]]

Codecov Report

Merging #303 (c3b0e60) into main (3fbf31a) will decrease coverage by 2.55%. The diff coverage is 52.38%.

:exclamation: Current head c3b0e60 differs from pull request most recent head 5df58f4. Consider uploading reports for the commit 5df58f4 to get more accurate results

@@            Coverage Diff             @@
##             main     #303      +/-   ##
==========================================
- Coverage   68.63%   66.08%   -2.56%     
==========================================
  Files         141      152      +11     
  Lines        7991     8970     +979     
==========================================
+ Hits         5485     5928     +443     
- Misses       1952     2400     +448     
- Partials      554      642      +88     

Flag	Coverage Δ
e2e	48.69% <0.00%> (-1.60%)	:arrow_down:
integration	57.95% <49.14%> (-1.81%)	:arrow_down:
unittests	48.38% <13.43%> (-2.03%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
plugins/teststeps/exec/ocp_parser.go	27.14% <27.14%> (ø)
plugins/teststeps/exec/runner.go	43.54% <43.54%> (ø)
plugins/teststeps/exec/exec.go	32.60% <44.44%> (ø)
...gins/teststeps/exec/transport/ssh_process_async.go	50.29% <50.29%> (ø)
...lugins/teststeps/exec/transport/local_transport.go	54.54% <54.54%> (ø)
plugins/teststeps/exec/transport/transport.go	57.14% <57.14%> (ø)
plugins/teststeps/exec/transport/ssh_process.go	61.81% <61.81%> (ø)
plugins/teststeps/exec/transport/ssh_transport.go	63.41% <63.41%> (ø)
pkg/test/param_expander.go	66.66% <66.66%> (ø)
plugins/teststeps/exec/events.go	66.66% <66.66%> (ø)
... and 4 more

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[mimir-d]

forgot a method rename in the last commit; the test was passing but it resulted in the controlled remote process hanging around because it wasnt actually killed (there is no wait for the controlled process to die, it just sends the sigkill)