dtoc: Move test files into a test/ directory

It is confusing to have the test files in the same places as the
implementation. Move them into a separate directory.

Add a helper function for test_dtoc, to avoid repeating the same
path.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_dtoc.py b/tools/dtoc/test_dtoc.py
index 6865d94..523f0a9 100755
--- a/tools/dtoc/test_dtoc.py
+++ b/tools/dtoc/test_dtoc.py
@@ -72,7 +72,7 @@
     Returns:
         str: Filename of compiled file in output directory
     """
-    return fdt_util.EnsureCompiled(os.path.join(OUR_PATH, dts_fname),
+    return fdt_util.EnsureCompiled(os.path.join(OUR_PATH, 'test', dts_fname),
                                    capture_stderr=capture_stderr)