Use hardcoded path for unittests

Index: src/gn/command_format_unittest.cc
--- src/gn/command_format_unittest.cc.orig
+++ src/gn/command_format_unittest.cc
@@ -21,7 +21,7 @@ using FormatTest = TestWithScheduler;
     std::string out;                                                        \
     std::string expected;                                                   \
     base::FilePath src_dir =                                                \
-        GetExePath().DirName().Append(FILE_PATH_LITERAL(".."));             \
+        base::FilePath("${WRKSRC}");                                        \
     base::SetCurrentDirectory(src_dir);                                     \
     ASSERT_TRUE(base::ReadFileToString(                                     \
         base::FilePath(FILE_PATH_LITERAL("src/gn/format_test_data/")        \
