Tests: utils: add gen-ust-nevents-str util

Tests: utils: add gen-ust-nevents-str util

This program generates a given number of UST events, each one having
in its payload the next command-line argument string in a cycle:

    gen-ust-nevents-str 10 hello world its me

will generate 10 events with the following strings:

    hello
    world
    its
    me
    hello
    world
    its
    me
    hello
    world

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>