Merge branch 'unit-tests'

Replace the "check" based libstrongswan unit test framework with our own,
giving us more flexibility for our specific needs.

The new framework is more portable and uses complete libstrongswan init/deinit
cycles for each test to properly catch leaks. It fully supports multi-threaded
tests, and brings many of them for all threading primitives, watcher and
streams.

The --enable-unit-tests option is not required anymore for libstrongswan tests,
but still is for the still "check" based charon-tkm tests.