Move a bunch of code out of testing/test_support.h.

They should either go into testing/utils/file_util.h,
testing/free_deleter.h, or testing/fx_string_testhelpers.h.
Then do IWYU and remove all the unnecessary test_support.h includes.

Change-Id: Idae09a08b96e2bc0b95ac1820ff305cdfa3f4e1a
Reviewed-on: https://pdfium-review.googlesource.com/c/49930
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/fpdf_attachment_embeddertest.cpp b/fpdfsdk/fpdf_attachment_embeddertest.cpp
index 6ab82f8..f2fd769 100644
--- a/fpdfsdk/fpdf_attachment_embeddertest.cpp
+++ b/fpdfsdk/fpdf_attachment_embeddertest.cpp
@@ -9,6 +9,7 @@
 #include "public/fpdf_attachment.h"
 #include "public/fpdfview.h"
 #include "testing/embedder_test.h"
+#include "testing/fx_string_testhelpers.h"
 #include "testing/utils/hash.h"
 
 static constexpr char kDateKey[] = "CreationDate";