[device] add test for UdevLoader::Get

Let me add a sanity check just to confirm UdevLoader::Get() works.
I will use this test to investigate loader behavior in msan on xenial.

Bug: 981731
Change-Id: I824f95767cb4ccdb73f544e8cea670571b1cc8d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697661
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676499}

device/udev_linux/udev_unittest.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "device/udev_linux/udev.h"
+#include "device/udev_linux/udev_loader.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -18,4 +19,8 @@ TEST(UdevTest, DecodeString) {
   ASSERT_EQ("E-MU Systems,Inc.", UdevDecodeString("E-MU\\x20Systems\\x2cInc."));
 }
 
+TEST(UdevTest, Loader) {
+  ASSERT_NE(nullptr, UdevLoader::Get());
+}
+
 }  // namespace device