build: fix nunit dll filename

Linux is case-sensitive with filenames, and as the NUnit library files
are lowercase, we need to use lowercase in the build files also.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@iki.fi>

build/Tests.lua

@@ -92,7 +92,7 @@ function LinkNUnit()
 
   links
   {
-    "NUnit.Framework",
+    "nunit.framework",
     "NSubstitute"
   }
 end

src/Generator.Tests/Generator.Tests.lua

@@ -20,6 +20,6 @@ project "CppSharp.Generator.Tests"
     "CppSharp",
     "CppSharp.AST",
     "CppSharp.Generator",
-    "NUnit.Framework",
+    "nunit.framework",
     "NSubstitute"
   }