Build unit test driver from the default target

We need an easy way to build the unit test driver without running the
tests. This enables passing options like --gtest_filter to the
executable, which can't be done very cleanly when running under
`make test`.

Fixed a number of compiler errors/warnings when building the tests
in various configurations by Jenkins.

Change-Id: I9198122600bcf02520688e5f052ab379f963b77b