test/cycles: restore default delay callback

test/cycles: restore default delay callback

test_delay_us_sleep registers sleep based delay for testing.
This changes the default delay function of testing environment.
It is not expected.

Restore default delay function after the test to fix the issue.

Fixes: a51639cc720a ("eal: add nanosleep based delay function")
Cc: stable@dpdk.org
Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>