Add a pixel test for axial shading

Add a pixel test for axial shading. This is a simplified PDF from the
provided PDF in pdfium:2001 and it can be rendered in various viewing
tools (including Acrobat reader) without having error messages.

The expectation is exported from Preview and it should be fairly close
to what the PDFium expectation will be after this issue is fixed.

Note: No .in file is uploaded for this issue because getting rid of
the binary streams in this PDF will trigger Acrobat to report errors.
You can find a copy of the .in file at [1].

[1] https://bugs.chromium.org/p/pdfium/issues/detail?id=2001#c6

Bug: pdfium:2001
Change-Id: I497845b220fedef81987689b8ab83c78f18a1772
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/105410
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Nigi <nigi@chromium.org>
diff --git a/testing/SUPPRESSIONS b/testing/SUPPRESSIONS
index b917743..bf49297 100644
--- a/testing/SUPPRESSIONS
+++ b/testing/SUPPRESSIONS
@@ -362,6 +362,9 @@
 # TODO(pdfium:1973): Remove after associated bug is fixed
 bug_1973.in * * * *
 
+# TODO(pdfium:2001): Remove after associated bug is fixed
+bug_2001.pdf * * * *
+
 # TODO(chromium:237527): Remove after associated bug is fixed
 bug_237527_1.in * * * *
 
diff --git a/testing/resources/pixel/bug_2001.pdf b/testing/resources/pixel/bug_2001.pdf
new file mode 100644
index 0000000..f73831d
--- /dev/null
+++ b/testing/resources/pixel/bug_2001.pdf
Binary files differ
diff --git a/testing/resources/pixel/bug_2001_expected.pdf.0.png b/testing/resources/pixel/bug_2001_expected.pdf.0.png
new file mode 100644
index 0000000..fd353a9
--- /dev/null
+++ b/testing/resources/pixel/bug_2001_expected.pdf.0.png
Binary files differ