WIP: tool for testing providers

[lutter]

This adds a new command line tool for running tests against a provider; tests are written in Ruby (mruby) and the command line tool serves as a driver that loads a simple test harness and the test itself. To run it, you need to do this:

cd build
make
export RALSH_DATA_DIR=$PWD/../data
export RALSH_LIBEXEC_DIR=$PWD/mruby/host
./bin/ral-check ../data/providers/tests/ssh_authorized_key.rb

The test file mentioned in the ral-check command shows what the test harness can do right now (not all that much)