[layer/test] Bug fix for dropout + unittest

[layer/test] Bug fix for dropout + unittest

This patch adds bug fix for dropout for training as well as inference
mode. Futher, added unittest:
- when dropout_rate is 0 or 100%, all the values are checked
- when dropout_rate between 0 and 100, weak check ensures that either
values are equal or one of the values (golden vs output) is 0
- when dropout_rate r between 0 and 100, strong check ensures that
100 - 2*r percentage of values must always match

All the checks for performed in the unittests.

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>