t6406: skip "external merge driver getting killed by a signal" test on Windows

The run_command() on the platform does not seem to report death by
signal as the caller expects.  For now, skip the test on Windows.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6406-merge-attr.sh

@@ -168,7 +168,7 @@ test_expect_success 'custom merge backend' '
 	rm -f $o $a $b
 '
 
-test_expect_success 'custom merge driver that is killed with a signal' '
+test_expect_success !WINDOWS 'custom merge driver that is killed with a signal' '
 	test_when_finished "rm -f output please-abort" &&
 
 	git reset --hard anchor &&