Move TestLoader into testing/test_loader.h.

Change-Id: Ifb3bfcf574fe3a2ee34946172850b19aa0534a39
Reviewed-on: https://pdfium-review.googlesource.com/c/49911
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 091d08a..69f5118 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -32,6 +32,7 @@
 #include "samples/pdfium_test_dump_helper.h"
 #include "samples/pdfium_test_event_helper.h"
 #include "samples/pdfium_test_write_helper.h"
+#include "testing/test_loader.h"
 #include "testing/test_support.h"
 #include "testing/utils/hash.h"
 #include "testing/utils/path_service.h"