Increase open files limit for unit tests

On machines with high CPU cores, few lib/ns unit tests fail due to not
enough file descriptors. Increase limit, it would be set higher on 40+
core machines anyway.

bind.spec

@@ -598,12 +598,15 @@ export TSAN_OPTIONS="log_exe_name=true log_path=ThreadSanitizer exitcode=0"
 
 %if %{with UNITTEST}
   pushd build
+  ORIGFILES=$(ulimit -n)
+  ulimit -n 4096 # Requires on some machines with many cores
   make unit
   e=$?
   if [ "$e" -ne 0 ]; then
     echo "ERROR: this build of BIND failed 'make unit'. Aborting."
     exit $e;
   fi;
+  ulimit -n $ORIGFILES
   popd
 ## End of UNITTEST
 %endif