Add test for ConcurrentSkipListSet#headSet(E, boolean)

There is already the test for the API, but was indirect.
This new test directly tests the API.

Bug: 182165523
Test: atest CtsLibcoreTestCases:libcore.java.util.concurrent.ConcurrentSkipListSetTest
Change-Id: I946b1493260297174782c59b8c7f7aff2b6fca86
1 file changed