systest: solution that will allow to write system tests in multiple packages

[dshulyak]

• current solution compiles single module and stores it is as a test binary
• it has a drawback that all system tests should be stored in the same module
• options:
  • include golang binary into image. during build phase compile them but don't run. during run phase execute tests with go run and rely on golang cache to avoid binary rebuild
  • checkout other test runners (such as ginkgo). they may allow to compile multiple test package into one testing binary.