Fix comparison between signed and unsigned error on darwin

mode_t is a uint16_t on darwin, which causes
sb.st_mode & ~S_IFMT
to produce an int when the uint16_t is promoted for the operator.
Cast to unsigned int before comparing against 0660U.

Change-Id: Ib1439c08d9e2b297eeeba701891508d269c19a3d
diff --git a/base/file_test.cpp b/base/file_test.cpp
index cbb2751..b138094 100644
--- a/base/file_test.cpp
+++ b/base/file_test.cpp
@@ -66,7 +66,7 @@
       << strerror(errno);
   struct stat sb;
   ASSERT_EQ(0, stat(tf.filename, &sb));
-  ASSERT_EQ(0660U, (sb.st_mode & ~S_IFMT));
+  ASSERT_EQ(0660U, static_cast<unsigned int>(sb.st_mode & ~S_IFMT));
   ASSERT_EQ(getuid(), sb.st_uid);
   ASSERT_EQ(getgid(), sb.st_gid);
   std::string s;