Octave testsuite: force gnuplot toolkit for text graphics.

This is necessary since FLTK is now the default toolkit in octave 3.8.

(cherry picked from commit c4a4e6eeb3a443d19a2045e31e82796b6f9ba1b1)
Ref. manual: fix default number of periods of forecast command.

(cherry picked from commit c768ff239b50e5ead38368424ced013f57d2be18)