Improve test coverage for PKIXParameters.

There's a lot more we could do here, but this covers APIs
used by Mainline modules.

Bug: 150638126
Test: atest CtsLibcoreTestCases:libcore.java.security.cert.PKIXParametersTest
Change-Id: I28d217ff5d6cbd76318871f4c4f3defa9c4917df
1 file changed