test: mark pseudo-tty/no_dropped_stdio as flaky

We've determined there is a test issue related to python as
opposed to node, mark as flaky until we can resolve

PR-URL: https://github.com/nodejs/node/pull/8385
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

test/pseudo-tty/pseudo-tty.status

@@ -0,0 +1,3 @@
+[$system==aix]
+# test issue only, covered under https://github.com/nodejs/node/issues/7973
+no_dropped_stdio           : PASS, FLAKY