Make sure the random test bitrate is high enough

The ForceIntraFrame test will fail (giving up after 100 skipped
frames) if the bitrate is not set high enough.

Set the minimum bitrate to w*h/50, which is a very low value,
but which still should allow getting a non-skipped frame within
a few attempts.
Merge pull request #1641 from mstorsjo/test-min-bitrate

Make sure the random test bitrate is high enough