Add sleep to ShouldReportStats Test in libinput_tests

Tests were flaky previously, by adding this sleep, this should should
assert that stats.shouldReport() is always true.

Test: atest LatencyStatisticsTest
Bug: 139050511
Change-Id: I9ca106fca758378eded19c6bf130aaee5387967c
Signed-off-by: Atif Niyaz <atifniyaz@google.com>