Refactor tests to simplify and inline test cases

[MattKotsenas]

Mechanical refactor as a prerequisite for improving test coverage and moving from a custom test harness over to the Microsoft.CodeAnalysis.Testing harness.

Each test class used the same procedure:

1. Split the Data\*.cs file into multiple files, one per test method
2. Update the test harness split into multiple [Fact]s, one per Data class and accept baselines
3. Remove unused code from the Data class and accept baselines
4. Inline the Data class into the test harness

The CodeFix test is a bit different. Because the diagnostics and code fix should be tested together, I inlined those Data classes into a string. A future refactor to use the Microsoft.CodeAnalysis.Testing harness will further clean those up.

Added a few TODOs where test coverage appears unnecessary or missing, but avoided adding it in this change due to the difficulty of review.

[rjmurillo]

Thanks @MattKotsenas