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>

nomad/api_querier_mock.go

@@ -7,6 +7,8 @@ import (
 
 	api "github.com/hashicorp/nomad/api"
 
+	io "io"
+
 	mock "github.com/stretchr/testify/mock"
 
 	url "net/url"
@@ -54,6 +56,27 @@ func (_m *apiQuerierMock) EvaluationStream(evalID string, ctx context.Context) (
 	return r0, r1
 }
 
+// ExecuteCommand provides a mock function with given fields: allocationID, ctx, command, stdin, stdout, stderr
+func (_m *apiQuerierMock) ExecuteCommand(allocationID string, ctx context.Context, command []string, stdin io.Reader, stdout io.Writer, stderr io.Writer) (int, error) {
+	ret := _m.Called(allocationID, ctx, command, stdin, stdout, stderr)
+
+	var r0 int
+	if rf, ok := ret.Get(0).(func(string, context.Context, []string, io.Reader, io.Writer, io.Writer) int); ok {
+		r0 = rf(allocationID, ctx, command, stdin, stdout, stderr)
+	} else {
+		r0 = ret.Get(0).(int)
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func(string, context.Context, []string, io.Reader, io.Writer, io.Writer) error); ok {
+		r1 = rf(allocationID, ctx, command, stdin, stdout, stderr)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
 // JobScale provides a mock function with given fields: jobId
 func (_m *apiQuerierMock) JobScale(jobId string) (int, error) {
 	ret := _m.Called(jobId)