Simplify build system for unit tests.

Simplify build system for unit tests.

- Now that gtest and gmock are build from bundled sources, there should
  be no reason to allow disabling just gmock from the tests (at least
  not strong enough reason compared to the complexity it brings to
  writing the tests and maintaining the build system).
- Make libxml2 mandatory for building any unit tests.  Again,
  simplifies the build system significantly.  Even before this change,
  very few of the unit tests were built without libxml2.

Closes #935.

Change-Id: I8ec5552661860ca43a00d70957fd82d69dcdf739