Fix utils.WaitForFdCondition inner retry loop

Commit dfdc4060 added WaitForFdCondition which uses utils.Retry without
handling timeout exceptions. This breaks any nested retry loops.

This patch fixes the above function, and also changes utils.Retry to
detect and warn future similar cases. In addition, we add a few small
unittests for utils.Retry.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>