Port libminijail unit tests to C++.

That way they can be run on Android Platform Continuous Testing,
which requires gtest for parsing of test output.

Fix one issue in free_label_strings(), which was not setting
|labels->count| to 0. Also fix some nits in test files.

Bug: 30973585
Change-Id: I8bdb414329aab82e2180d412b9ecc128beab7227
8 files changed