Normalize filepath inputs in Test Execution Protocol

TEP does not currently define how filepaths should be defined, most urgently this affects windows vs. unix paths with `\` vs. `/` used as path separators.

First thoughts at a scheme for normalizing:
* Runner is responsible for normalizing the file path separators
* Passes filepaths with `/` used as separator to adapters so adapters can assume unix-style
* Will this work when invoking on windows? We'll have to test, maybe adapters will have to each handle path separator explicitly

The issue names no files or tests. Start by reading the Test Execution Protocol and the runner-to-adapter filepath boundary, then test both Windows and Unix path separators. Done means the protocol defines the normalization behavior and adapters receive consistent paths without breaking Windows execution.

Written by the indexing model from the issue text.