test: py: Add an option to skip sleep test

Some QEMUs have a problem with time setup that's why
sleep test is failing. Introduce env__sleep_accurate
boardenv variable to have an option to skip sleep test.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>