Add delay to deflake mainline tests

MdnsIPv6LinkLocalWithDefaultRoute and IPv6LinkLocalWithDefaultRoute are
flaky on APCT mainline testing. The new added route or interface address
somehow does not apply immediately. If the DNS resolver call connect()
right after that, EADDRNOTAVAIL can be returned.

Added a short delay after adding route or interface address to avoid
flaky.

Bug: 247693272
Bug: 252944188
Test: 300 runs on ABTD, all passed
Change-Id: Ie3192aad11bea205ad9bc5611a6a391f35fff4a0