Normalize filepath inputs in Test Execution Protocol

[msrose]

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