Run unit tests for the Rust track of Exercism.

See:

• docker.md
• interface.md

• The test runner itself is a simple shell script: run.sh
• The rust project in this repo is a filter which transforms Cargo's native testing output to the format expected by Exercism
• The dockerfile contains a multi-stage build: the first stage builds the test script, and the second stage uses it to run the student's solution and test suite and transform the output appropriately