sunrpc/tst-udp-*: Fix timeout value

Following on 00c3da43 ('sunrpc/tst-udp-timeout: Fix timeout value').

While e.g. 2.5 timeout can be represented exactly with doubles, time stamps
may not be represented that exactly, and thus with coarse-grain clocks (e.g.
10ms) we may end up with 2.499-ish values due to rounding errors.

* sunrpc/tst-udp-timeout.c (test_udp_server): Add more timeout slack.
* sunrpc/tst-udp-nonblocking.c (do_test): Likewise.