testsuite: add valgrind suppression

Problem: a new valgrind test failure was encountered on aarch64,
Ubuntu 20.04.2 LTS and also the official Jetson Ubuntu 18.04:

==1705645== Syscall param epoll_ctl(event) points to uninitialised byte(s)
==1705645==    at 0x4BDFE38: epoll_ctl (syscall-template.S:78)
==1705645==    by 0x48B37EF: epoll_modify (ev_epoll.c:96)
==1705645==    by 0x48B4F57: fd_reify (ev.c:2166)
==1705645==    by 0x48B4F57: ev_run (ev.c:3677)
==1705645==    by 0x48B4F57: ev_run (ev.c:3623)
==1705645==    by 0x48824FF: flux_reactor_run (reactor.c:126)
==1705645==    by 0x1113BF: main (broker.c:449)
==1705645==  Address 0x1ffefff22c is on thread 1's stack
==1705645==  in frame #1, created by epoll_modify (ev_epoll.c:72)

Since this is apparently internal to libev, add a suppression.

Fixes #3808

t/valgrind/valgrind.supp

@@ -123,3 +123,13 @@
    fun:hwloc_topology_load
    ...
 }
+{
+   <issue_3808>
+   Memcheck:Param
+   epoll_ctl(event)
+   fun:epoll_ctl
+   fun:epoll_modify
+   fun:fd_reify
+   fun:ev_run
+   ...
+}