sigchain: add test.

Add a gtest for the various sigchain wrappers.

Bug: http://b/73344857
Test: adb shell /data/nativetest64/art/arm64/art_sigchain_tests/sigchain_test
      (with several actual failures)
Change-Id: I331da4f44008984723e2e2733e2a6dd66658e704
3 files changed