Make paths case-sensitive on Unix

[rkoeninger]

Fixes #321

• Added properties bool CaseSensitive, StringComparer Comparer, StringComparison Comparison to MockFileSystem, IMockFileDataAccessor. The other Mock* classes use these properties to perform string comparisons and to opt to do ToUpper/ToLower or not.
• MockDirectory.GetLogicalDrives() returns drive letters in upper case instead of lower case.
• Moved HasIllegalCharacters, CheckInvalidPathChars from MockPath to PathVerifier as it's the more appropriate place.
• Added IsWindowsPlatform() to MockUnixSupport.
• Removed the optional Func<bool> argument to methods on MockUnixSupport that overrides platform detection. Tests that need to run on a specific platform are labeled with the relevant *Only attribute.
• Added/updated tests as needed. Tests that specifically asserted case-insensitive behavior were labeled WindowsOnly. Similar tests were added to assert case-sensitive behavior and are labeled UnixOnly.

Fixes #395

• Added special case handling to MockDirectory.GetParent for Unix to make sure a slash-root parent is properly identified and returned.
• Added a test for this, labeled UnixOnly.

Fixes #405

• Fixed MockDirectory.GetFiles to handle un-normalized slashes:
  • Use of alternate slashes.
  • Missing or present trailing slash.
  • Redundant slashes.
• Added cross platform test for this.

[rkoeninger]

@fgreinacher @ericnewton76 thoughts?

There's also a comment in there about a pre-existing thing that I had a question about.

[fgreinacher]

Wow, that’s a lot of improvements, thanks a lot! Please give me one or two days to have a look at this.

[ericnewton76]

Mine are just nitpicks.

I assume this works correctly, although there's several lines touched that didn't otherwise need be.

[fgreinacher]

The changes look great! I left some nit-picky comments, will merge as soon as those are addressed!

[rkoeninger]

@fgreinacher Covered everything listed so far. Let me know if you see anything else.

[fgreinacher]

Perfect now!

[fgreinacher]

Thanks again for all your work on this @rkoeninger 🎉