JDK 9 RFR of JDK-8075111: Mark testFlatMappingClose (from CollectorsTest) as serialization hostile

[Amy Lu]

Testcase testFlatMappingClose was newly introduced in JDK-8071600 in 
CollectorsTest, this test should be marked as serialization-hostile 
(such tests will ignored by by lambda serialization testing framework).

bug: https://bugs.openjdk.java.net/browse/JDK-8075111
webrev: http://cr.openjdk.java.net/~amlu/8075111/webrev.00/
Thanks,
Amy