Add arbitrary wait to allow request to go through

This test may be flaky due to a race condition when checking that
the request went through. Introduce an arbitrary wait of 1 second
as a temporary fix.

Test: atest ExplicitHealthCheckServiceImplTest
Bug: 146767850
Change-Id: I3ce812fd4b82f35a9135e82000e1d17654754791
Exempt-From-Owner-Approval: I'm owner in main watchdog directory, this is a trivial change
1 file changed