tests: drop -d option from ffserver invocation

It randomly causes failures with an error like:
"Failed to set value '-f' for option 'd': Error number -920332800 occurred"

(The error number is different every time.)

Reviewed-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>