clar has historically shelled out to `/bin/cp` to copy test fixtures
into a sandbox.  This has two deficiencies:

1. It's slower than simply opening the source and destination and
   copying them in a read/write loop.  On my Mac, the `/bin/cp` based
   approach takes ~2:40 for a full test pass.  Using a read/write loop
   to copy the files ourselves takes ~1:50.

2. It's noisy.  Since the leak detector follows fork/exec, we'll end up
   running the leak detector on `/bin/cp`.  This would be fine, except
   that the leak detector spams the console on startup and shutdown, so
   it adds a _lot_ of additional information to the test runs that is
   useless.  By not forking and using this internal system, we see much
   less output.