Adding test cases for enter key press on buttons

Adding embedder test cases and pixel test for enter key press on check
box and radio buttons.

Bug: pdfium:1431
Change-Id: Ia8fb9658fa0770407fc9b044733aeb335814240f
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/66410
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/pwl/BUILD.gn b/fpdfsdk/pwl/BUILD.gn
index ea26c13..ea2ce94 100644
--- a/fpdfsdk/pwl/BUILD.gn
+++ b/fpdfsdk/pwl/BUILD.gn
@@ -50,6 +50,7 @@
   sources = [
     "cpwl_combo_box_embeddertest.cpp",
     "cpwl_edit_embeddertest.cpp",
+    "cpwl_special_button_embeddertest.cpp",
   ]
   deps = [
     ":pwl",