Reduce number of lint warnings

When compiled against the SDK, junit.framework is noted as deprecated.
Changing this to use JUnit4-style asserts doesn't break any users.

Test: cts-tradefed run cts -m CtsLibcoreTestCases
Change-Id: Ic480b16db8e0ef13b55e7f3c005c7d9c26da0114
2 files changed