Add a regression test in the RRL system test

Add a regression test in the RRL system test

This commit modifies the MTU of the loopback interface on
Linux systems to 1500, so that oversized UDP packets can
trigger EMSGSIZE errors, and tests that named handles
such errors correctly.

Note that the loopback MTU size has not yet been modified
for other platforms.

(cherry picked from commit cfd058d622102c18ea9b6ef9cd37530d62debea7)