Enable some kind of gradual typing in test_runner (#222)

It's not realistic to enable full-blown type checks
within test_runner's codebase, since the amount of
warnings revealed by mypy is overwhelming.

Tests are supposed to be easy to use, so we can't
cripple everybody's workflow for the sake of imaginary benefit.

Ultimately, the purpose of this attempt is three-fold:

* Facilitate code navigation when paired with python-language-server.
* Make method signatures apparent to a fellow programmer.
* Occasionally catch some obvious type errors.

test_runner/fixtures/utils.py

@@ -1,13 +1,15 @@
 import os
 import subprocess
 
+from typing import Any, List
 
-def get_self_dir():
+
+def get_self_dir() -> str:
     """ Get the path to the directory where this script lives. """
     return os.path.dirname(os.path.abspath(__file__))
 
 
-def mkdir_if_needed(path):
+def mkdir_if_needed(path: str) -> None:
     """ Create a directory if it doesn't already exist
 
     Note this won't try to create intermediate directories.
@@ -18,7 +20,7 @@ def mkdir_if_needed(path):
     os.mkdir(path)
 
 
-def subprocess_capture(capture_dir, cmd, **kwargs):
+def subprocess_capture(capture_dir: str, cmd: List[str], **kwargs: Any) -> None:
     """ Run a process and capture its output
 
     Output will go to files named "cmd_NNN.stdout" and "cmd_NNN.stderr"
@@ -42,7 +44,7 @@ def subprocess_capture(capture_dir, cmd, **kwargs):
 _global_counter = 0
 
 
-def global_counter():
+def global_counter() -> int:
     """ A really dumb global counter.
 
     This is useful for giving output files a unique number, so if we run the