Unit tests execute very slowly in Debug (i.e. unoptimized) builds. This
causes TravisCI to terminate the job due to no console output being
seen.
To workaround this for the debug builds the tests are just compiled
but not executed. To implement this the `RUN_UNIT_TESTS` environment
variable now can take on the values `BUILD_ONLY`, `BUILD_AND_RUN`,
and `SKIP` rather than `0` or `1`.
defaults. The location is dependent on the implementation.
This triggered a build failure on TravisCI because the location
of the default Python binary is different to what is in the Docker
container.
