• Adds a builder option in groups, that lets a user override the builder used by a test,
• Adds a build_config.path option for docker:go and docker:generic that lets a user define a subfolder for their test.

We provide ARG PLAN_PATH to the docker:generic container as a helper (a user might also set the value themselves through build config options).

With these two features, we can define a test that builds go and rust code, or go and node, or node, and rust, etc.

Fixes #1357 and #1305

Example of composition.yml

[global]
  plan = "integrations_mixed_builders"
  case = "issue-1357-mix-builder-configuration"
  total_instances = 2
  runner = "local:docker"

[[groups]]
  id = "go"
  instances = { count = 1 }
  builder = "docker:go" # override builder here

  [groups.build_config]
    path = "go/" # define base path here

  [groups.run.test_params]
    expected_implementation = "go"

[[groups]]
  id = "generic"
  instances = { count = 1 }
  builder = "docker:generic" # override builder here

  [groups.build_config]
    path = "generic/" # define base path here
    build_args = { image = "golang:1.16-buster" }

  [groups.run.test_params]
    expected_implementation = "generic"

Tasks

• adds integration tests
• update the composition & groups to allow builder options
• update the builder & runner to support multiple builders during a task
• support a path option in docker:generic and docker:go to is a subfolder as the test source root
• take one more look at the path option support & try to simplify implementation in docker:go
• move a few integration tests out of the pkg folder until we tackle EPIC: Speed up development with improved integration testing tooling (no more shell scripts or flakiness) #1368