Test framework organisation overhaul.

- CKANTests and NetKAN are root notes.
- Test classes are named after the code they exercise.
- Directory structure changed to reflect this.
- Test data now in `TestData.cs` for ease of re-use.

For #90
Added tests for IsKspDir() and test data.

Flipped IsKspDir() to internal so we can test it.

Three new data directories:

- `KSP/KSP-0.25` - A "good" install of KSP 0.25
- `KSP/bad-ksp` - A directory with no version info
- `KSP/missing-gamedata` - A 0.25 install with no GameData dir.

For #90