[backend] fix warnings in 0015-BSSched-EventSource-Directory.t

without this patch you get warnings like this:

t/data/0015/events/x86_64/unblocked::devel:languages:perl::openSUSE_Leap_42.1: No such file or directory at t/0015-BSSched-EventSource-Directory.t line 264.

after successful completion of all jobs.
This is missleading because $! is _not_ reset and not meant to detect an error.

src/backend/t/0015-BSSched-EventSource-Directory.t

@@ -259,8 +259,11 @@ push(@files_to_remove,"$ev_dir/$file_name");
 unshift(@files_to_remove,"$eventdir_base/$arch/$evname");
 
 for (BSUtil::unify(sort {$b cmp $a} @files_to_remove)) {
-  ( -d $_ ) ? rmdir $_ : unlink $_;
-  warn "$_: $!" if $!;
+  if ( -d $_ ) {
+    rmdir $_ || warn "$_: $!"
+  } else {
+    unlink $_ || warn "$_: $!";
+  }
 }
 
 exit 0;