With the switch to `netstandardapp1.5` framework for the host, the xUnit
test runner broke. For `dotnet test` to work with the .NET Core xUnit
runner, it must continue to target `dnxcore50`. Fortunately, we can
import our project's framework, and make everything work again.
Also improved the `xunit.sh` script to check for dependencies.
Unfortunately a wrapper is needed because `LD_LIBRARY_PATH` must be set
to avoid having to place build artifacts (specifically
`libpsl-native.so`) out-of-tree to run tests.
