UBIFS: improve power cut emulation testing

This patch cleans-up and improves the power cut testing:

1. Kill custom 'simple_random()' function and use 'random32()' instead.
2. Make timeout larger
3. When cutting the buffer - fill the end with random data sometimes, not
   only with 0xFFs.
4. Some times cut in the middle of the buffer, not always at the end.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>