dtoc: Use pathlib to find the test directory

Update this so that the directory being used is declared at the top of
the file. Use pathlib as it seems to be more modern.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_dtoc.py b/tools/dtoc/test_dtoc.py
index 0f544f9..597c93e 100755
--- a/tools/dtoc/test_dtoc.py
+++ b/tools/dtoc/test_dtoc.py
@@ -13,6 +13,7 @@
 import copy
 import glob
 import os
+import pathlib
 import struct
 import unittest
 
@@ -28,7 +29,8 @@
 from u_boot_pylib import test_util
 from u_boot_pylib import tools
 
-OUR_PATH = os.path.dirname(os.path.realpath(__file__))
+DTOC_DIR = pathlib.Path(__file__).parent
+TEST_DATA_DIR = DTOC_DIR / 'test/'
 
 
 HEADER = '''/*
@@ -91,7 +93,7 @@
     Returns:
         str: Filename of compiled file in output directory
     """
-    return fdt_util.EnsureCompiled(os.path.join(OUR_PATH, 'test', dts_fname),
+    return fdt_util.EnsureCompiled(str(TEST_DATA_DIR / dts_fname),
                                    capture_stderr=capture_stderr)