tests, testutils: separate unit tests from tools
testutils: add communication with the test executor (WIP)

Two of the steps that `make check` will follow are now implemented (set up
and tear down the test environment), but there are still remaining:
1) Send the app + tests to the test executor virtual machine
2) Ask the test executor to run all the tests
3) Gather the test results from the test executor

These steps will be implemented as a python API with the functions
send_app(), run_tests() and retrieve_test_results().