CodeRunner/Runtime: add failure cases to test suite

[camerondurham]

This may be blocked by #15

Create test suite of "malicious" code to understand limits of the CodeRunner. This will be helpful for benchmarking and understanding how much progress we've made toward the Code Isolation Milestone.

These test cases can be wrapped in an environment variable check so they do not fail the build and just used in a temporary test container. Example:

func Test_MaliciousCode(t *testing.T) {
  if os.Getenv("TEST_MC") != "" {
    // test stuff that may intentionally break
  }
  // rest of the test
}

• add set of test cases for the runtime that run separately from unit tests (ideally in Docker)
• test failure modes
  • throw an unexpected exception
  • spawn many processes
  • "escape" the current directory
  • spawn a shell
  • ... (think of other ideas)