llvm/test/lit.cfg: Retweak for Win32 to fix testing.

  - execute_external should be;
    - Not on Win32.
    - Using bash.
    In reverse, "execute_internal" shoud be (Win32 && !bash).

  - lit.getBashPath() behaves differently before and after tweaking $PATH.

I will add a few explanations there later.

llvm-svn: 159641