Rewrite Clang resource directory lookup logic in test driver.

Due to this code we ended up multiple Clang resource directories in the path which is a problem because Clang's stdint.h uses an include_next<> logic and it was getting messed up with the multiple headers in the lookup path.

src/Generator.Tests/GeneratorTest.cs

@@ -45,12 +45,22 @@ namespace CppSharp.Utils
             var path = Path.GetFullPath(GetTestsDirectory(name));
             options.addIncludeDirs(path);
 
-            var headersPaths = new System.Collections.Generic.List<string> {
-                Path.GetFullPath(Path.Combine(path, "../../deps/llvm/tools/clang/lib/Headers"))
-            };
+            var foundClangResourceDir = false;
+            for (uint i = 0; i < options.SystemIncludeDirsCount; ++i)
+            {
+                var dir = options.getSystemIncludeDirs(i);
+                if (dir.Contains(@"lib/clang/"))
+                {
+                    foundClangResourceDir = true;
+                    break;
+                }
+            }
 
-            foreach (var header in headersPaths)
-                options.addSystemIncludeDirs(header);
+            if (!foundClangResourceDir)
+            {
+                var dir = Path.GetFullPath(Path.Combine(path, "../../deps/llvm/tools/clang/lib/Headers"));
+				options.addSystemIncludeDirs(dir);
+            }
 
             driver.Diagnostics.Message("Looking for tests in: {0}", path);
             var files = Directory.EnumerateFiles(path, "*.h");