[tests] Purge tests and start with a clean implementation of subprocess

Seems like subprocess and friends (Popen, communicate) are not so easy
to mock cleanly. Therefore, start from scratch and carefully write test
by test, until (at least) the old test coverage has been restored.

bumblebee/util.py

@@ -15,6 +15,8 @@ def execute(cmd, wait=True):
         out, _ = proc.communicate()
         if proc.returncode != 0:
             raise RuntimeError("{} exited with {}".format(cmd, proc.returncode))
+        if type(out) == str:
+            return out
         return out.decode("utf-8")
     return None