Add Collectors.mapping tests.

Cover Collectors.mapping with unit tests.

Bug: 182165513
Test: atest libcore.java.util.stream.CollectorsTest
Change-Id: I17439ccf648b5db8d30715d46e41e451b931b78c
(cherry picked from commit bd2717711271290403670a32de295b446875d5ac)
Merged-In: I17439ccf648b5db8d30715d46e41e451b931b78c
1 file changed