Tests: fix: leak of notification-client arguments

Valgrind reports the following leaks:

==334435== HEAP SUMMARY:
==334435==     in use at exit: 8,568 bytes in 24 blocks
==334435==   total heap usage: 846 allocs, 822 frees, 1,425,964 bytes allocated
==334435==
==334435== 9 bytes in 1 blocks are definitely lost in loss record 1 of 11
==334435==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==334435==    by 0x4BCC5BE: strdup (in /usr/lib/libc-2.33.so)
==334435==    by 0x10A48B: main (notification-client.c:123)
==334435==
==334435== 13 bytes in 1 blocks are definitely lost in loss record 2 of 11
==334435==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==334435==    by 0x4BCC5BE: strdup (in /usr/lib/libc-2.33.so)
==334435==    by 0x10A473: main (notification-client.c:120)
==334435==
==334435== 27 bytes in 1 blocks are definitely lost in loss record 3 of 11
==334435==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==334435==    by 0x4BCC5BE: strdup (in /usr/lib/libc-2.33.so)
==334435==    by 0x10A444: main (notification-client.c:114)

Free the strdup()'ed' arguments.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I21523e847141aef50858d3018f37f0125dee0e2c