sunrpc: Properly clean up if tst-udp-timeout fails

The macro TEST_VERIFY_EXIT is used several times on
sunrpc/tst-udp-timeout to exit the test if a condition evaluates to
false. The side effect is that the code to terminate the RPC server
process is not executed when the program calls exit, so that
sub-process stays alive.

This commit registers a clean up function with atexit to kill the
server process before exiting the main program.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>