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