Copy core-junit files into legacy-test.

The legacy-test directory contains part of the Android API and
so must maintain backwards compatibility. The junit classes that
are in the API are copied here to ensure that they do not change
when external/junit is upgraded.

Bug: 30188076
Test: make checkbuild and checked legacy-test contents
Change-Id: I6c263afc16c06abfd027334be51a41422239a189