when running tests, include whatever args or env vars were set in the scheme.

Xcode lets you pass extra arguments or environment vars to your tests
when they're run.  By default, it'll pull whatever you've configured in
the Run action of the scheme editor.  Optionally, it can also pull these
from the Test action of the scheme editor.

Let's make sure these args or env vars get passed through to the tests.

This is partial fix for issue #34.  Xcode also allows these arguments or
environment variables to include macro expansions from the test target's
build settings.  For instance, you could set the env var
'MyConfiguration' to '$(CONFIGURATION)' and it will do the substitution
for you.  We'll tackle that in another change.