Fix case-sensitive path issues on Linux

I just tried, and it uses a different version of NUnit in `csharp/MarkdownSharpTests`, which doesn't understand `-framework:4.0`:

```
mono  /auto/users/robnea/dev/FSharp.Formatting/csharp/MarkdownSharpTests/nunit-console/nunit-console.exe "-nologo" "-noshadow" "-labels" "/auto/users/robnea/dev/FSharp.Formatting/tests/FSharp.CodeFormat.Tests/bin/Release/FSharp.CodeFormat.Tests.dll" "-xml:TestResults.xml" "-framework:4.0" 
Unhandled Exception:
System.ArgumentException: The requested value '4.0' was not found.
  at System.Enum.Parse (System.Type enumType, System.String value, Boolean ignoreCase) [0x00000] in <filename unknown>:0 
  at NUnit.Core.RuntimeFramework.Parse (System.String s) [0x00000] in <filename unknown>:0 
  at NUnit.ConsoleRunner.ConsoleUi.MakeTestPackage (NUnit.ConsoleRunner.ConsoleOptions options) [0x00000] in <filename unknown>:0 
  at NUnit.ConsoleRunner.ConsoleUi.Execute (NUnit.ConsoleRunner.ConsoleOptions options) [0x00000] in <filename unknown>:0 
  at NUnit.ConsoleRunner.Runner.Main (System.String[] args) [0x00000] in <filename unknown>:0 
```