ci: skip an unreliable test on macOS

Sometimes, this test fails on macOS, at least on the GitHub CI.
It looks as if the fping process is killed before it produces
the required output.  This test works reliably on GNU/Linux, so
keep it, but skip it when running on macOS.

ci/test-07-options-i-m.pl

@@ -51,7 +51,10 @@
 }
 
 # fping -l -Q
-{
+SKIP: {
+if($^O eq 'darwin') {
+    skip 'On macOS, this test is unreliable', 2;
+}
 my $cmd = Test::Command->new(cmd => '(sleep 2; pkill fping)& fping -p 850 -l -Q 1 127.0.0.1');
 $cmd->stdout_is_eq("");
 $cmd->stderr_like(qr{\[\d\d:\d\d:\d\d\]