Remove flaky test (depends on Vim's initial state; very awkward)
Bug fix for xolox#misc#path#tempdir() (E739 was not being caught)
Make external command execution work properly without vim-shell!

In vim-easytags/issues#58 it became apparent that external command
execution on Windows using vim-misc without vim-shell was completely
broken :-(

I never noticed before because I always have vim-shell installed and
honestly just never gave the interaction any thought... Crap.

This commit fixes xolox#misc#os#exec() on Windows when vim-shell is
not available. It also adds testing infrastructure to run all tests
involving xolox#misc#os#exec() twice, once with vim-shell disabled
and then again with vim-shell enabled (assuming it is available).

For more details, see the issue page on GitHub:
  https://github.com/xolox/vim-easytags/issues/58