Make the test runner understand Windows command shell execution.

Currently, the test runner makes the assumption that it will run
commands through /bin/sh.  This is obviously not true on Windows,
so this patch abstracts this logic out somewhat.  Instead of
having the caller build the command string himself, the caller
will now pass in argument list of the form [[a, b], [c, d], ...]
which will get converted into a string of the form a b; c d or
a b && c d, depending on the platform.

Reviewed by: Todd Fiala

Differential Revision: http://reviews.llvm.org/D4590