This test verifies that the implementation correctly detects all of
the errors specified in the "Errors" section of the
EXT_transform_feedback spec, with the following exceptions (which seem
like they should be handled in separate tests):

- Errors related to BeginQuery and EndQuery.

- Error due to mismatch of transform feedback mode and drawing mode
  (e.g. drawing GL_LINES when transform feedback is expecting
  GL_TRIANGLES).

- Errors due to exceeding MAX_TRANSFORM_FEEDBACK_SEPARATE_ATTIBS in
  a call to TransformFeedbackVaryings, GetTransformFeedbackVarying,
  or
  Get{Integer,Boolean}Indexedv(GL_TRANSFORM_FEEDBACK_BUFFER_BINDING).

v2: Modify "not_a_program" test to create a program and then
immediately delete it.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>