Add tests and end-to-end tests for websocket execution

For unit tests, this mocks the runners Execute method with a
customizable function that operates on the request, streams and exit
channel to simulate a real execution.

End-to-end tests are moved to the tests/e2e_tests folder. The tests
folder allows us to have shared helper functions for all tests in a
separate package (tests) that is not included in the non-test build.

This also adds one second of delay before each end-to-end test case by
using the TestSetup method of suite. By slowing down test execution,
this gives Nomad time to create new allocations when a test requested a
runner. Another solution could be to increase the scale of the job to
have enough allocations for all end-to-end tests.

Co-authored-by: Maximilian Paß <maximilian.pass@student.hpi.uni-potsdam.de>

environment/job_test.go

@@ -55,7 +55,7 @@ func createTestJob() (*nomadApi.Job, *nomadApi.Job) {
 	base := nomadApi.NewBatchJob("python-job", "python-job", "region-name", 100)
 	job := nomadApi.NewBatchJob("python-job", "python-job", "region-name", 100)
 	task := createTestTask()
-	task.Name = fmt.Sprintf(TaskNameFormat, *job.ID)
+	task.Name = TaskName
 	image := "python:latest"
 	task.Config = map[string]interface{}{"image": image}
 	task.Config["network_mode"] = "none"