Unit test Vagrant::Util::Subprocess's STDIN support
Unit test Vagrant::Util::Subprocess's STDIN support

- create new unit test file for this class, as none existed.
- test `Vagrant::Util::Subprocess#execute` behavior relating to STDIN
  handling.

mwrock/vagrant,taliesins/vagrant,crashlytics/vagrant,taliesins/vagrant,marxarelli/vagrant,mitchellh/vagrant,mitchellh/vagrant,jtopper/vagrant,signed8bit/vagrant,gitebra/vagrant,jtopper/vagrant,jtopper/vagrant,benh57/vagrant,mitchellh/vagrant,bryson/vagrant,lukebakken/vagrant,lukebakken/vagrant,justincampbell/vagrant,marxarelli/vagrant,benh57/vagrant,bryson/vagrant,signed8bit/vagrant,sni/vagrant,bshurts/vagrant,bdwyertech/vagrant,gitebra/vagrant,signed8bit/vagrant,jtopper/vagrant,marxarelli/vagrant,mitchellh/vagrant,sni/vagrant,justincampbell/vagrant,lukebakken/vagrant,crashlytics/vagrant,signed8bit/vagrant,justincampbell/vagrant,mwrock/vagrant,sni/vagrant,bshurts/vagrant,chrisroberts/vagrant,crashlytics/vagrant,lukebakken/vagrant,PatrickLang/vagrant,bshurts/vagrant,gitebra/vagrant,bshurts/vagrant,benh57/vagrant,benh57/vagrant,bdwyertech/vagrant,chrisroberts/vagrant,sni/vagrant,bdwyertech/vagrant,chrisroberts/vagrant,PatrickLang/vagrant,bdwyertech/vagrant,gitebra/vagrant,mwrock/vagrant,marxarelli/vagrant,PatrickLang/vagrant,bryson/vagrant,crashlytics/vagrant,chrisroberts/vagrant,justincampbell/vagrant,PatrickLang/vagrant,taliesins/vagrant,mwrock/vagrant,taliesins/vagrant,bryson/vagrant