test_sleep: move test_sleep from tests/legacy/kernel to tests/kernel

As test_sleep does not have dependency over legacy APIs.
So moving files from tests/legacy/kernel to tests/kernel.

Jira: ZEP-2009

Change-Id: I2439391ba6d0a194d07a0d1b48911d37b2f493b0
Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>