regtest: suppress console output from tests

This disables all console output from the *-regression.sh scripts by
default.  If the V environment variable is set to 1, ffmpeg commands
are printed before being run.  If V is greater than 1, ffmpeg output
goes to the console, otherwise stderr is redirected to a file.

Originally committed as revision 21253 to svn://svn.ffmpeg.org/ffmpeg/trunk