subtract_test: drop '_t' from local typenames
subtract_test: drop '_t' from local typenames

_t is reserved by posix

+ switch to camelcase
  http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Type_Names

Change-Id: Ic738b97d492f244c58989a0f73a4d763340ec274

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